I just bought a Tascam US-122 mk-II USB audio interface, and I'm having an odd problem with it. When I plug my electric guitar directly (no amp) in to the line/guitar jack and set the switch to guitar mode, I get some popping/crackling static noises while I play. Not severe, but I can hear them on recordings. If I set the switch to line-in mode and turn up the gain, I don't get any unwanted noise. Is this a common problem with these things, am I doing something wrong, or is my unit defective?