audio scratching sound problem

I have an USB M-Audio Sonica and I'm having some trouble during audio
playback. I am periodically getting a loud scratching noise from the sound
output of the card. The pattern seems random, and I assume that the scratch
sound comes when the computer is processing, but I have monitored all file
access and memory access and I can find no correlation between the
scratching and the computer accessing any files or memory.

I also have another USB soundcard that I can hook up and I will not have
this problem at all. So I know that the problem is with the actual sonica
and I assume that the audio signal is getting interfered with somehow.

Furthermore in my testing I have still had the problem when starting in safe
mode, or in diagnostic mode in XP. I have even unplugged everything from
the laptop and started the computer in safe mode and I still have the
problem.

Thanks!