[Elspode]

Power supplies on both computers are plugged into a grounded plugstrip which is in turn plugged into a grounded outlet. So the system is grounded. The only other connection is from each computer's headphone out jack to L/R line ins on a receiver/amp.

I have also bypassed the plug strip and plugged the computers directly into the wall socket with the same result. In reading up, ground loops aer not completely unknown, hence, the existence of the device I posted. I am quite familiar with 60 cycle hum in audio gear. DI boxes, for example, usually have a 'ground lift' setting, and other audio gear, such as mixer boards, usually have a polarity invert switch which does essentially the same thing.

I'm just kinda baffled why it is happening in two separate laptops, yet does not occur in anything else which comprises the audio path (stereo, tv, etc).