I experienced this exact same issue on my Pioneer 4100NEX. When I was on the Bluetooth source, and activated the dimmer setting either through the Dimmer Settings Menu or the Picture Adjustment Menu, I would experience a very high pitched noise coming through the speakers. It was annoying enough that I could hear it over the music while driving at night.
I contacted Pioneer Technical Support and the technician was able to reproduce the issue on his 4100NEX as well. After a couple of weeks of troubleshooting on their end, they admitted it was an issue with the 4100NEX and sent me out a 4200
