Background hum when recording

I’m using a Peavey USB 6 connected to my Macbook Pro and a Behringer condenser mic. I’m also using Audacity 2.0. When I test the mic through Audacity using monitor microphone everything sounds crystal clear. However, when I start recording I get a love volume, high pitched hum. It not only can be heard through the headphones, it also shows up on the recording. I can get rid of it easily with noise removal too. When I quit recording the hum quits. I’ve tried this with only battery power thinking it might be a ground problem. No difference. And when I record with Sound Studio 4, no hum, which leads me to think it’s Audacity.
Seems like I might be missing an easy setting so I’m putting it out there.

I’m using Lion OSX 10.7.4 and I installed Audacity from a dmg file