I just had a new compressor and then new right front shock installed on my 04 A8. Now when the compressor runs, which seems to be 2-3 times in the first few minuets of running, the compressor is incredibly loud. It is a new Wabco unit I bought from Germany (ebay $-zahn)and is OEM. I feel there is a problem with it so I have rebuilt the original ( i6 automotive kit ) and will install soon. Thing is, I never heard the compressor before so I don't know if it is running too much, as though there was a leak in the system. As I said, I replaced a shock (Arnott from carID) so it is hard to imagine there is yet another leak. Perhaps this is expected behavior from the compressor?

 

Generally, the compressor is fairly quiet on an A8. Check to be sure the rubber isolators are good and the pump isn't touching anything it shouldn't be. That would transmit the normal vibration and noise into the body.
One thing I do recall is replacing the solenoid valve block for leaking a few times. The usual complaint was excessive compressor operation after the car sits OR it drops overnight. It's behind the left front wheel well liner.
Of course, if the compressor is really loud, could be it's not working as it should. Normally, it may run a few seconds after start up but shouldn't time out and have to run 2-3 times to level the car. There's a thermal protection switch that trips to keep the compressor from running too long and overheating.

 

Thanks Aspen. My mechanic has assured me the rubber isolators are in place and good. Perhaps this weekend I will give him the rebuilt compressor to put back in. I will personally inspect the bushings at that point. Hopefully this will take care of the excessive noise. As for the frequent running I hope to check as many air connections for leakage as possible while in there. It sounds like the solenoid block is in the area so that will be a good target. I guess it is possible that when the compressor ran silently it was running when it should. Sometimes what you don't know ....