I got a Polk iSonic for my computer. When attached to my Mini through either audio out or via usb DAC it squeals when set to AUX at 38 +-5 db around 1K hz. Same from my wife's Mini audio out. Her MB and my MBP have no such problem. This is simply with no signal from the computer, i.e. nothing playing. Her Mini is a Core Duo, mine is a recent i7. Laptops 4 and five tears old. Any idea why, and how to fix it?