high pitch whistle noise when let off gas coasting when car is warmed up

[stephenxchen]

greetings to all Audi Gurus.

I posted before, but, I want to describe this more clearly this time.

Mine is 2007 A6 C6 4F engine.

this problem started about 3 months ago, still haven't been able to fix it, hence here seeking for help.

this high pitch whistle noise only occurs when the car is about fully warmed up, when let off gas paddle coasting, no difference if in gear or in neutral. seems getting louder right before stop, sometimes still can hear it for a few seconds or a few minutes after the car has stopped.

so far, I have replaced Idler Pulley, Belt Tensioner, Serpentine Belt, Water Pump.

Noise stops as soon as I remove the oil cap. There is a lot of strong suction force when I remove the oil cap.


no codes, no check engine lights, everything is perfectly fine except the noise. all fluids levels are fine, no leaking, no vacuum leak either.


I am at wits end.

any ideas, suggestions? please mechanically explain why you think so.

[chuckee98]

PCV valve may need to be replaced. My '08 S6 was doing the same thing. Dealer confirmed defective PCV valve, and replaced it. When I went to pick up the car, noticed that the whistling was still there. Turns out that there was another vacuum leak causing the whistle. Car is still in the shop to replace the intake manifold due to a multitude of leaks where the two halves meet, and carbon cleaning of intake valves since the intake manifold is already off.

[95blkmax]

^^ what chukee98 said! It makes sense that it gets worse when off throttle and engine at operating temp (highest possible intake manifold vacuum conditions). If it's not the PCV box (sits under the intake manifold) itself, it will be with the lines that come off of it (dual inlet, one from each valve cover... and one output into the bottom of the IM elbow right behind the TB).