Dynamo Hum

I am going crazy trying to hunt down the source of an audio hum in my Mac system. The moment I plug the Firewire cable into my DV converter, I get that distinctive 60hz audio buzz, and "hum bars" in my video. It's obviously a grounding problem somewhere, I've rewired everything possible but it's still humming. Even if my Mac and DV box are turned off, I get the hum on my regular TV. This is some weird, deep interference, I'm stumped.
On a brighter note, I did manage to resurrect my old 9Gb hard drive with my mp3 collection on it. The drive just needed a little dusting and replugging the connections, and it fired right up.