I have noticed this myself. I think has to do with the way the variable line-out volume patch works.

Ah yes, I remember now. When I first took the empeg VolAdj code and patched the Receiver kernel ( http://www.norman.cx/receiver/ ) I also tried to apply the variable line-out volume patch at the same time. I noticed that it added hum to the output, but when I mentioned it here no one else seemed to have the problem. I couldn't put up with it, so I removed the line-out patch from my kernel.