First of all, the intercom should be grounded directly off of your battery. Or at least from a grounding strip that comes right off the battery. It is very important that as you add farkles, that ALL grounds come right off the battery. This is the best way to keep from creating ground loops in your system.
Also, what other audio devices do you have plugged into your StarCom that may be introducing a ground loop issue? Any device that is being powered from the bike's electrical system has an opportunity to introduce a ground loop issue, and therefore you'll hear the buzzing in your headset.
FWIW, I have my GPS, cell phone, iPod, FRS/GMRS radio, and Valentine One all powered from the bike and into my intercom system. And all without the lovely buzz that you're experiencing. But EVERY ONE of these devices has a GLI (ground loop isolator) cable between the device and my intercom system.
Two final notes: 1) I have not had any success cleaning up unwanted noise by cleaning up the power side of the equation. Removing the noise from the audio cables is the only thing that is "tried & true" for me. 2) I recently cleaned up some unwanted static from my Valentine One's audio be rerouting it's audio cable. Made a HUGE difference.
Hope this helps...