Forgive me, this is my second engine noise question from a recently bought '02 A6. This occurs cold with the engine at 2k rpm or less when the engine has been pulling slightly and I let up on the throttle but not completely. It sounds like a light rapid knock which I can get to continue if I manage to hold the throttle at that position, still on the gas slightly from the pulling the engine was doing. If I let up too far I hear the noise briefly but have passed through the spot where it occurs. Even in a close garage I can't hear it if I'm just revving the engine to the same rpm w/o the pulling part. I have to be doing this on a slight hill or a flat road and withinin the first few minutes of driving from fully cold. It's very throttle sensitive. A harmonic? Something more serious?
I tried leaving the car at my Audi dealership overnight to demo the noise to a tech the next morning, but unbelievably, I couldn't get it do make the noise. Annoying

Thanks,

John