What cartridge are you using? If it happens to be a very high output cartridge like some Grados, they can cause hum.
Turntable Hum
I've got an old Technics that is giving me fits...there's a slight hum present, that's there regardless of whethter the unit has a headshell attached or not, and regardless of whether it's AC power is plugged in. When I swing the arm across the platter, the hum varies from crackling and loud to nothing. It's not near a transformer or anything like that. Any ideas? I'm really perplexed by the behavior of the arm.