I just recently bought this 2.8 quattro A6...and have noticed that whenever my A/C is on this loud nise comes from what I think is the fan. It's like a loud jet noise I guess and it turns off right when I turn off the A/C. Can anybody tell me what it is.

 

ac compressor, most likely, it takes the hot air from outside, compresses it, cools it, and expands it over a set of coils that hot air passes over from outside and comes through your vents

its run off of your engine, if its unberably loud, take it to the dealer or mecanic and have them check it out

ps, i this ish, from pimp my ride?

 

All of the Audis must do it. I have a 2001 4.2 A6 and everyone comments on how loud it is when the AC is on. It really does sound like a jet engine. My cars nick name is the Bat Mobile at work. Is this normal? I don't want to take mine to the dealer! They will tell me some BS and charge something for nothing.

 

Think you will find its the fan, had exactly the same prob, local Audi guy told me bearings gone in fan , showed me all the play in it.
Bad news you have to have the whole fan renewed, here in the Uk it cost me £190 ($1.81) to the £.

 

I had the same problem with my 2001 A6 2.8. It sounded like a siren. I checked with the dealership and found out it was a secondary fan to allow the increase of air flow into the car. They wanted $450 for the part plus labor.
I found the part here:

http://catalog.eautopartscatalog.com...tion=000573068


for $235.16 and payed a local mechanic $50 to put it in. It's as quiet as now as when the air is turned off

hope this helps