Very annoying hum on my USB interface...

[Yousef]

Okay I have an Alesis multimix 8USB. I've had it for around 3 years now and I've been recording with it and it sounds great and has no noticeable lag.

What I've always done is monitor the audio through separate USB headphones from my soundcard. Well, my heaphones broke ($10 on ****) and I got new headphones that came with a 3.5mm to 1/4 adapter, so I said hey, I'll try and monitor everything through the mixer!
Everything works fine, EXCEPT, there is this hum through the headphones when I have set it as going to master so I can hear everything on the computer. Its really annoying and gets louder as I put increase the volume. I can barely hear it when something is playing, but when nothing is playing this annoying hum is present.

Any solutions? The hum is not present when I have it switched to monitoring the microphone, only when its going to master so I can hear the stuff on the computer, so I think its probably a computer hardware/software issue, and not a problem with the mixer.

Thanks.