I just bought a 97 A4 about 6 months ago, but during the past month I noticed this really loud noise coming from the engine when I start the car, it is really loud at low RPMs but dies down once I get past 2500 RPM. I had a mechanic check it out and he could not find anything wrong, but this sound is really annoying, can anyone tell me what it might be ? Thanks

 

When was your water pump last replaced? Any idea where it is coming from in the engine? Any way you can get an audio clip of the sound and post it?