I have the noise too, it's high pitched and constant, does not change with RPM, etc.
sounds like the ringing that you hear after coming out of a rock concert. I noticed it more after driving for awhile, it gets louder. Probably b/c every electrical component has warmed up. It's definitely electrical. I thought it was the HID lights, but it's still there if you turn the lights off. It's coming from right under the dash somewhere, but have not been able to pin point it. Might be a grounding issue or wire shielding issue of some sort. Another problem I have is that when my cell phone is in the center console or cup holder, I get the dreaded chatter through the speakers, just before receiving a call or text message. So I know their is a wire shielding issue there, but not sure if it is related.
