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I have a 2002 Mustang GT and it is making an extremely loud high pitched noise noise near the intake manifold. I am throwing the following codes P1506, P0171, P0174, P0316, P0307, and sometimes P0303 and P0300 comes and goes. I thought it was the IAC valve so I replaced it but nothing. Then I checked the vacuum lines and added some hose clamps where needed and teflon tape where needed, and still nothing. Next, I sprayed WD-40 around the vacuum lines to see if there was a change in rpms but nothing again. Lastly, I retorqued the intake bolts, upper plenum bolts, and throttle body bolts, and again nothing. Please someone help me. Also here's a video of the problem. Https://www.youtube.com/watch?v=-EuFK-P5kz4&feature=youtu.be
 

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Has the cold air intake been on the car or did you recently install it? Has any work been done to the motor/ car recently? Disconnect the IAC and watch for rpm change. If rpms don't change much IAC valve is not working properly. If they drop check for vacuum leaks, make sure all hoses and boots on CAI are on good and tight. If you have a multimeter check that you have 5V reference going into the IAC, make sure you have voltage coming out of IAC (at idle voltage should be less than 1V and should sweep as motor is reved), also check ground is being supplied. A vacuum leak can be the cause of your P1506, P0171, and P0174 codes. Leans codes could also be related to your misfires. Check/change your spark plugs, and swap a couple coils around too. Clear the codes, and if the misfire codes come back on different cylinder then you know it's the coil pack.
 

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I ask about the air intake because some intakes flow so much air the MAF cannot read it properly and will throw off the IAC. Maybe the whistling is an alternator bearing or something going out. Take off the serpentine belts, start the car for a minute and see if the noise goes away. If it goes away then you know it's an accessory, tensioner, or idler pulley.
 
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