Try flipping the AC plug-ins around in their sockets one at a time. If your on split phase with ground return this could help. Let me know if this helps and I will explain why it does.
Grounding Hum
I recently got a new TT for Christmas. It's an MMF 2.2 LE. I think there's a grounding problem or something because there is a slight buzzing noise present. I looked through past threads on the topic here, but nothing seems to fit this situation. I've checked, and rechecked the connection between the TT, and the preamp. I've even used a Fluke voltmeter to check for continuity between the two. I've checked the headshell leads to the cartridge. All is well. Maybe it's the ground wire connection to the turntable itself? I've run out of options. Any ideas?