Hi, my 2005 Audi A4 has a knocking noise coming from the front of the engine. My local Audi garage says it is the aircon compressor and they want £633 plus labour to fix it.

The aircon is fine, I always have it on auto. The very strange thing is this:
1. It has been this way for 2 years since I bought it.
2. Coincidentally when I took it to the garage to show them, I turned on the engine, got the noise for about 4 seconds, then it stopped! Now, I am not sure if it has ever done that before (because I have just learnt to ignore it) but I am usually aware of it every time I start the engine.
3. Now for the weird bit. I turned off the aircon and the noise came back immediately. I turn it on and the noise goes away!

Any ideas?