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Discussion Starter #1
My 2004 Mustang GT has a whine/humming sound coming from the rear end only under slight acceleration or when holding a steady speed. It starts making the noise around 45 mph and will continue to about 55-60mph. I typically drive around 50mph most of the time and this is when the noise is most apparent and loudest.

I took a video with my cell phone to capture the audio. I have uploaded the video to youtube but I'm not sure if it's okay to post the link here or not.
 

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Why guess? Pull the rear diff cover and inspect it. Catch the oil in a clean pan. Inspect for metal debris. Rear diff's are at their heart simple devices. You would be surprised what can be learned from a "visual" inspection.

IMO, the odds favor a bad bearing. The only question is which bearing. The carrier and wheel bearings are fairly easy to replace. The pinion bearing......well.....not so easy.
 

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Did the gears get changed? Was the driveshaft ever changed? Was the "dog bone" removed? Universals good?
 

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Discussion Starter #4 (Edited)
Did the gears get changed? Was the driveshaft ever changed? Was the "dog bone" removed? Universals good?
I have no idea if the gears or driveshaft were ever changed before I got the car. What is the "dog bone" that you speak of? Last time I checked the u-joints they were fine.

I also have this clunking sound in the rear end if I hit the accelerator hard or when I come to a complete stop without feathering the brake pedal.

EDIT: Okay I know what the dogbone is now. It's there, but I recall back in Feb. when I had it on a lift it was kinda loose/wobbly. We ended up trying to tighten it down with some makeshift rubber and what not, it's tighter than it was but still not as it should be. Should I remove it?
 

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Discussion Starter #5
Why guess? Pull the rear diff cover and inspect it. Catch the oil in a clean pan. Inspect for metal debris. Rear diff's are at their heart simple devices. You would be surprised what can be learned from a "visual" inspection.

IMO, the odds favor a bad bearing. The only question is which bearing. The carrier and wheel bearings are fairly easy to replace. The pinion bearing......well.....not so easy.
I would rather not pull the cover off unless I know exactly what's wrong. Why pull the cover and have to fill it back up with oil only to have to pull again and fix at another time. I have no intentions of doing the work myself. I can do some things but I don't even want to attempt a repair like this myself.
 

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The "cluncking" noise when you step on the gas could very well be your universals.

The "dog bone" is a weight under / near the rear pumpkin just in front. Its there for harmonics stopping vibration. People remove it and hear and feel nothing while others do. You could remove it if you want. May stop some of the noise you are hearing because its loose. Mine was very tight before I removed it, this was years ago.

Check your vin number and find what gear you should have. Then jack the car up spin the tire and count the rotation of the tire to the driveshaft. This will give you a very close idea of what's is in your pumpkin. The driveshaft visual is easy to tell...if looking like its aluminum, its probably been changed.

I agree with wmburns, you have to look to in there to see what's wrong...if that's your problem. If you are going to pull the differential cover off (and you do have stock gears) might as well replace the gears, bearing, pinion, etc while you are there.
 

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Discussion Starter #7 (Edited)
Driveshaft is original. As far as I know/can tell the car is bone stock, not even a CAI. Next weekend I will have the car on a lift and will check things out a bit more closely.

I don't have access to the tools to change bearings or gears and I don't know anyone that does. I also have no idea how to perform that kind of work. I would like to get the rear end rebuilt regardless and have some 3.73 or 4.10 gears installed, but I don't know a place to take it that isn't going to try to charge me over $1k.

I can get a rebuild kit, axle bearings, and gears for around $300, I just need someone that can do the work for me at a reasonable price that will allow me to bring my own parts.
 

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Discussion Starter #8
So, I changed the Diff fluid today. Fluid was dirty, but there were no metal chunks or anything in it, everything looked fine inside as far as I can tell. Filled it up with Royal Purple and some Friction Modifier. Still got the rear end hum/whine mostly prevalent around 50mph. What should I check next?

Also I found a small amount of play in my upper and lower control arms on the passenger side but don't have sockets big enough to tighten down, will have access to a lift and more tools tomorrow. Think that could be the cause of my rear end clunking? The u-joints and yolk don't seem to have any play in them either.

I also have a creaking in my front suspension now. I hope it's not a ball joint! I know both of my inner tie rods are bad and plan on changing them this weekend, really don't want to have to do a ball joint and locating all the tools I need.
 

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Did you happen to check the great ratio when you had the cover off? Also, how many miles are on the car? Did you wiggle the ring gear up and down? If there is a lot of play when you do that, part of your ring gear could be chewed off... and by part, I mean so little you would never see it, but you would hear it. Need more info to help lol
 

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Discussion Starter #11
I know absolutely nothing about rear ends, nothing looked broken though. Didn't really try to wiggle anything around. Didn't look at the gear ratio, about 130k miles
 

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yeah if they are in fact stock, then the ring and pinion could be worn...

your idea of gettting 3.73s is a great idea so i definitely think that would be a very good idea and could very well fix your issues but again you gotta find a competent shop to do so. there is too much involved to explain it all over the internet as to how to calculate and figure everything out. someone mentioned the driveshaft rotation trick so that should give you some kind of idea as to where you are for gear ratio. also to try to explain everyting about setting gears is too tough to explain withou showing you on your car.

so when you DO find a place to do gears, i suggest replacing your wheel bearings at the same time... and from experience of replacing them at least 5 times in 2 months, ONLY GET THE TIMKEN REPLACEMENT BEARINGS!!! each time i got that "bearing and seal all in one kit" and as previously mentioned i kept replacing the bearings.

gears are a hot commodity but apparently trying to find a competent shop, is quite a feat of strength lol i wish you the best of luck on your journey!!
 

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Discussion Starter #13
Ford racing bearings and seals arent any good? Think I'll go to Steeda in Valdosta, GA, only 2 hours away.
 

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I'm pretty sure that's who I bought the bearings from, and they came in as timken bearings lol
 

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Discussion Starter #15
Had the car on a lift today. Found that the transmission mount is shot, and all the bushings for the rear control arms are shot. Could literally move the the transmission up and down with my hands by the driveshaft and could move the whole rear end up and down with my hands from the axles. I couldn't locate a mount anywhere and I'm going to have to order one, that will be the first thing I change. Ordering some aftermarket control arms for the back as well and replacing the upper differential bushings. I didn't see any leaks at the axle bearings but there was a very slight play on both rear wheels up and down but I'm not sure if that is normal or not. I'm hoping all the slack I have in the driveline from the transmission all the way back to the control arms is possibly causing the whine, since I'm sure the pinion angle is way off. Anyone have any thoughts?
 

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im not so sure bushings make the noise rather than just squeaking... pinion angle would only snap off the driveshaft on a hard launch sooo i dont see how that would make a whine, but it sounds like it has been like that for a while. but put all your new stuff on and see what happens!! i hav been wrong a few times lol

if everything in the case is loose, then yeah, pinion angle could be your issue but that still means its something inside the rear end housing...
 

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Do NOT use the so called axle-saver bearings (bearing and seal in one unit). I have bad experience with them, among with several others I have talked to. Only get bearings directly from Ford or NAPA. All the Ford dealers in my area get their bearings from NAPA if they don't have them in stock, according to Ford it is the same manufacturer. I do not remember the exact brand, but I think mine were Timken also.

You should NOT have any up or down movement/play in your rear wheels. If you can physically move the wheels ever so little without the whole axle assembly moving, your bearings or axles are likely bad. I had to replace both my rear axles and wheel bearings as the previous owner had somehow twisted one of the axles, which did cause a slight whine, but the whine came from my brakes as the twisted axle had sheered one of the two caliper bolts right off, so I had to replace my rear brakes as well. I doubt that's the case for you, but as mentioned, if you have any play at all in your wheels you need to replace the bearings, and there is a chance your axles might be getting worn too at that point and should be replaced. Replacing the axles and bearings is not hard, but if you plan on taking it to a shop anyways that will be a good time to do gears as well. :bigthumbsup
 

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your noise kinda sounds like mine people keep telling me its the pinion bearing and I get the clunking noise when i let off and accelerate again you find out the problem?
 

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Discussion Starter #19
Not yet, I've been on vacation and waiting on a bunch of parts to arrive. This weekend I will have the car back on a lift again and replacing motor mounts, tranny mount, a arms, inner and outer tie rods, steering rack bushings, struts, upper and lower control arms, upper axle bushings, and axle bearings. I will report back for clunking and whine/hum after installation. Chances are I'm going to end up getting the rear end rebuilt and gears installed though.
 

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Discussion Starter #20
Replaced transmission mount and all rear control arms. Rear end is much tighter and has no play now. It still clunks a bit sometimes if I left off the gas and then immediately floor it. I did not remove the quad shocks, could the quad shocks possibly be causing the clunk? They do have a bit of play in them as the bushings are worn and I know they are no longer needed with the upgraded control arms.

Rear end still whines but I haven't changed axle bearings yet, and I know now that the drivers side axle bearing is leaking and I'm pretty sure that's where the whine is coming from as well.

Looks like ujoints may be leaking a bit of grease also possibly causing clunking as well? Definitely feels like it's near the wheels though. I will be changing them next along with the axle bearings and a new transmission seal at the driveshaft it looks like it's slinging fluid/grease on the frame above.
 
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