AnnieA

My A3 has an intermitant high pitched whistle which comes from the back of the engine compartment on the drivers side. The noise can be from a cold start or occur part way through a journey. The pitch sometimes changes or the noise disappears if I press the throtle. Strangely the noise can continue loudly for 30 secs or so after the engine is switched off, and then sounds like an aircraft powering off after landing. Despite the noise being a daily occurance now, my car seems to have a second sense and is never making the noise when I get to the Audi dealer so they are unable to offer any constructive advice

Lyman_T

Since one of the symptoms is that it winds down after the ignition is turned off, it has to be either air or water pressure being released or vacuum dissipating.

Were I more of a betting man, I'd say it could be a leak in the seal for the cruise control air cannister (if Audi uses one on their cruise control setup). Only a couple other places that a vacuum or air or water leak could cause the whistling after the engine was off.

Steam (pin-hole leak from a hot water line)
Steam (loose cap for radiator overflow unit)
Vacuum leak around the feed line to the master brake cylinder (not likely)
Turbocharger pressure release

Good luck, perhaps someone else will be able to give you a more complete picture. Somewhere to start, anyway.

bbbobbb

Here is what my Audi dealer did when I drove in with the whistle:

1) Do not turn off the vehicle
2) Put it in park with the engine running
3) Pop the hood
4) Carefully open the oil filler valve, if it blows like crazy (pressure is equalizing...) then you win a new PCV valve...

Here is more to read, item #2 on the list: http://forums.vwvortex.com/zerothread?id=3366758

BBC

You need a breather valve, engine case holds vacuum now not a separate reservoir. the leaking valve is venting off crank case vacuum while running and continues to do so after engine is shut down.