It may be a bad microphone. I would try returning it to the store where it was purchased for another one. If they can't take it, you can send it to Plantronics to replace. All you need to do is package the headset up and include your return address, phone number and a copy of your receipt and send to:
ATTN: Service Center
345 Encinal Street
Santa Cruz, CA 95060
I am more than confident it is not a bad mic but this is actually an xbox 360 issue.
Just from what you have said I am guessing you are using the Wired 360 controller or are playing with the 360 charge cable connected with the controller. The squealing/buzzing you are hearing is USB feedback from the XBOX controller and all headsets suffer from this, not just Plantronics.
Use a wireless controller.
Hope this helps