Microphone/cable for voiceovers

[aliasroks]

I'm sure you're all tired of hearing someone ask for microphone selection assistance. I'm using Lexicon Alpha audio interface/preamp with MacOS X 10.4.10 and Adobe Soundbooth. The mic I have is a Shure SM58. Not exactly sure about the microphone cable, though it says "Professional Low-Noise Microphone cable." I'm trying to use it for voiceovers, but while it's good about not picking up a lot of background noise, there is a high-pitched hum that I can't seem to get rid of. When I use the "noise cleanup" feature in Soundbooth it removes the hum somewhat, but ends up adding a subtle "transformer" like quality to my voice. Not good for voiceover work.

I'm rather new to this game and was wondering if anyone could give me some advice on what may be causing the hum (It's not the picking up sounds from my computer because I've got the mic and preamp across the room.) and how to either eliminate it or select equipment that will help prevent a hum.

Thanks