Though I haven't read the empeg FAQ and stuff yet, I think it's alternator whine

The empeg FAQ has an extensive discussion of ground loops and alternator whine, including links to several online documents that detail the necessary troubleshooting steps. Before discussing noise problems here on the BBS, make sure to go through all of the diagnostic steps listed in all of those documents. Be thorough, because the smallest detail could be the thing that's causing the problem.