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My 2001 Mustang GT (35.000 miles) has a whine sound from the rear axle differential. I had 3.73 gears installed a few years back, and I think the sound appeared shortly after that, however it has gotten worse over the years.

The sound starts from approximately 20 mph and remains at a constant high-pitch whether I go 40 mph or 70 mph.

The sound will increase slightly in dB the warmer it is outside, whereas when it is cold, it is much quieter. Also, it doesn't appear right when I start driving (from the engine being cool), but it will after a few minutes.

I have replaced both the rear differential fluid and friction modifier, but it didn't help.

My guess is that the gear install was never calibrated correctly as it seems like the sound appears due to the threads expanding when they warm up. What do you guys think? Would I need the gears re-calibrated?

Unfortunately the car is no longer in the States as I brought it with me abroad, and there are practically no mechanics with Mustang experience around, so I am out of luck taking it to a reputable place in the States.

I am currently in the States and would like to get any parts needed here to bring back, as they are twice as expensive abroad. What do you think I should get to fix the issue? Rear axle lubrication and friction modifier for the disassemble/assembly, this kit or something else? I'm trying not to spend too much and can only carry so much back, so I can't go for a complete rebuild package.

Thanks for any advice!
 

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I've found that the solid rear axles in Fords tend to be noisy as they age. I switched to Royal Purple in my factory rear after 100k miles and it helped a lot.

Since yours started making noise soon after the new gears were installed they were probably not installed correctly. Based on your cold and warm ambient temperature experience I'm guessing that the clearances are too tight. It may be a matter of removing a shim or installing a thinner one.

Your Haynes manual has a trial and error procedure, using dye, to obtain the correct clearances. You can also do this to diagnose wear.

Solid rear ends are used in delivery trucks overseas, so a truck mechanic there should be able to do this for you.
 
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