Here is a thread where we plowed much of the same ground discussed in the current thread, but also dug into the guts of the system in question (not a laptop with external power supply, but a desktop with the same noise issue) to try to better understand the source of the problem.
http://forum.cockos.com/showthread.p...ht=ground+lift
For reasons that are not totally clear to me, plugging the PC into the power outlet through an Ebtech Hum X Voltage Hum Filter (which appears to filter out ANY signal on the ground line)solved the problem. Indeed, the system had even less electrical noise than when a ground lift was used, and it is a totally safe solution.
T