This is also just more common with aftermarket head units. They pick up interference way easier than a stock one. You could try getting some waterproofing like they sell for when you put an aftermarket head unit in your boar. It helps on blocking the interference. Its just like a film type sticker thing that goes on your head unit.
The way I had it explained to me was this. You can get electronic static through more than just your antenna. It can actually be coming from the electronics inside the head unit. This is why your always supposed to run signal wires and power wires down different sides of the car. Mine was more from the motor. You could hear it change pitch as the motor changed RPM.
I asked our local stereo "installer" what would cause this and that is what he told me. I can't say I have scientific proof.
He told me to try the whole water proofing strip trick and to try to ground the case of the CD player to the car.
Before I got around to doing that I put in a whole new head unit (birthday present yay!) and this new kenwood does not have the interference problem.