That is caused by the car power system. It happens with car lighter adapters too. When the engines RPM raises so does the charge from the alternator, then to whatever powers your music. Check to make sure all your grounding is properly done. I think that should do it. I had a lighter adapter for my portable CD player and I had the same symptoms as you do. I wouldn't think the head unit would do this, I would check the source selector adapter.
Hope this helps some