If I powered the united through the ac/dc adapter only, there was complete silence although the unit was ready for play in all respects. But when the USB was plugged in, it made a hell of a racket--whining group loop hum and static. Using it on the desktop was obviously important to me--I don't like squatting down and doing all my work on the floor with a two inch LCD. I want the luxury of sitting and controlling my sound from a big screen. After trouple shooting and even trying two different computers, I found I could not abide the noise from the USB power and sent the unit back.
This is just an advisement of things to look out for if you're going to buy an expensive guitar effects processor. If it uses USB power--even if you can plug in grid power at the same time, you may experience noise that is quite a nuisance. You might want to make sure it just uses batteries or ac adapters. I spoke to techs at a competitor and simply asked if they used USB power, and they explained to me that it causes noise. I could attest to that myself. Just sharing.