Sorry to bump an old thread, but I recently bought an HS40 and have this exact problem.
What was causing the majority of my buzzing was my G25 steering wheel, plugged into the mains but not to the PC via USB. When I took the power out of the wheel the buzzing dropped dramatically, to a level that my buddies on Skype could tolerate lol.
It's a great headset sound quality wise but this issue lets it down so I'll have to return this one and find another.
Edit.. I ran an earth from the back of the pc to the metal on my steering wheel and all feedback has gone, bodge fix but works!