I have an issue with my USB 2.0 buss-powered Tascam US-122L where the line out ports pick up a whining noise whenever the laptop (a Toshiba U300) power supply is plugged in. Disconnect the PSU and the problem disappears. Try a different wall socket - same issue. Try a different laptop and PSU - same issue.
Any ideas? Should I have bought an interface with its own power supply?
It turns out that it was a ground loop problem, solved by any of the standard solutions. It's interesting to note that ground loops don't always result in low pitched hum, and can be 'digital' sounding (scratches, whining etc) in some instances.