Just started hearing a high-pitched whine or buzz from the front end of the car (2002 A4 3.0 tip). It only happens at specific RPMs - about 1400, 1800 and 2200.
It almost sounds like a slipping belt - but maybe more like a bad bearing in a fan or something? Anybody have this before?
I'm on a long trip so it will be a royal pain if I have to take the car in.
Thx!

 

It does sound like either a serpentine belt or a belt tensioner thingy. Both are normally done at the same time, try putting some lithium grease or belt dressing on the belt see if it makes any difference, if it does is the belt, if it doesn't is the tensioner.

 

I have that same problem, its either the tensioner or a bearing in AC/Alt, etc...

Low RPM's only. Need to change the belt and all that, should go away.

 

My '02 does the same thing at start up for about 30 to 40 seconds. It is small motor on the RH side of the car below the air box and seems to be plumbed to the turbo somehow. I have put my hand on it when it is running. Definitely an electric motor of some kind. What is it?

I have not looked in the service manual yet.

 

Grrr - I just had the timing belt done and they were supposed to do the serpentine belt and tensioner and water pump, etc, etc.
Will get in there and see what's going on.
Thanks everybody for the replies.

 

That's your secondary air pump. Is completely normal for it to make that noise.

 

What does this air pump do? And why does it only run for awhile?

Thanks for your help. It is always a pleasure to read your answers that you give.

 

Well, I'm not completely sure what it does here but I know part of its purpose is to stabilize the idle when your engine is cold. The main output of the pump goes directly into your airbox.

In my old MKIII Jetta it served as the main purpose for the doorlocks and trunk release.