Nool,

All logic dictates that your problem is with the mic. Even a new mic is not necessarily good. Do as Tony suggested, and get the mic checked out by someone at a sound shop. Also ask him the correct way to hook up the mic and what the mic connection requirements are. These items are not necessarily evident to someone who is not a sound technician (which is most people using microphones). Sometimes mics work great, and sometimes they become real bears. Don't give up.

Peter

Catsailor ... Before purchasing a Sennheiser off e-bay (sounds great BTW), I did some shopping around in local music shops ... In one shop, the rep wanted to demo an EV, an AKG and a Sennheiser ... the EV, fresh out of the box did not work!!! ...
I'm no sound tech, but I've learned through years of experience that even the obvious may not be the case...
t.

http://www.ebtechaudio.com/he-2des.htm
This will get rid of the hum (take note any live sound engineers this is the most effective/usefull bit of gear I own !!)
But i really think that if you got one you'd be just be fixing the symptom and not the cause .Smells like a bad earth somewhere in your setp up .
Are all your devices plugged into the same power point ??