Buzzing from engine bay
 

My car has developed a buzzing over the last couple of days. I could hear it from the cabin but, but when I opened the bonnet, the sound was much louder.

I have found the location, but I have no idea what the part is.

I have a 4.2 Petrol non-FSI engine. The part is under the rear engine cover and is like a disk that has a pipe up to it, and a pipe coming out of the side. The noise is like a high-pitched vibration. If I hold the part I can stop the sound - but what is this part?

edit - If my explanation is not good enough, here is an image

PCV valve by the looks of it, probably leaking due to failed diaphragm. Easy enough to replace.

I concur - may also be gummed up. Changed the one on our a6 a year or 2 back. Not too horrific on price from memory

About £25 including shipping from the USA for one I replaced a while back (on a D2).
I then dismantled the old one and cleaned it, so I have a reasonable spare, though the diaphragm isn't perfect, but does work OK.
Had to buy a tool to unclip it. Clic-R pliers I think they're called.

Thanks - I'll get one ordered, looks fairly easy to replace.

I had exactly the same problem just fixed it 7 days ago.. and on shutting down the engine it hooted.
Solution ...

A flat head take off the jubilee clips and then separate the casing and simply scrub all parts and spring ect with hot soapy water ..
Dry it out .. re assemble and lube up with WD40

And problem solved

I guarantee it will work ! +++

Well, as the car was in for its alloy refurb and it was spending a week on jacks, I got the garage to replace the valve.

All is much quieter now. It is surprising how little it takes for you to question it an A8, when in many of my other cars, it just happens to be one of many noises, and sometimes these things go unnoticed.

apparently is a crankshaft vent. I replaced my one 2 weeks ago as i couldn't listen to the whistling noise anymore...it sounded like AC on full blast. Easy to replace at £23 cost.