I had exactly this issue, and discovered a way to fix it myself, no tools or even skill needed. What fixed it for me was popping the volume control disc (on the right ear cup) off the base and making sure the entire area is clean. It's held on by some material that seems to wear with use and heat and in my case a small amount of the material had become wedged between the visible circuitboard and volume knob. As soon as I removed the stickiness, both the static and volume differential between ear cups disappeared at all volume levels.
Do beware, once you clean out whatever sticks the disc on...there will be nothing to hold the disc on. This isn't desirable as obviously dirtiness causes issues for this device. I would suggest using a weak super glue to reattach the disc to the knob once cleaning is finished, but BE CAREFUL not to get ANY glue on the circuit board. Just a bit on the top of the knob to hold the disc in place will be enough to protect the internal workings from further contamination. Again, this is extraordinarily easy to fix, but please, think before you act, as it is quite possible to make things worse if you were to, say, glue the volume knob to the circuit board.
My apologies for resurrecting an old thread, but as this was the second hit on google for "sennheiser pc 360 static", I thought it might be seen by others having this same problem.