2002 Audi A4 SE

I have this irritating problem, when the radio isn't tuned I get this irritating beep. I only noticed it when I tried to use an FM transmitter to play my mp3 player over the radio. I've tried all the settings but not joy. Anyone have any ideas?

 

does it do it when the volume is down? if so it's probably a bad ground some place.

 

It beeps the same whether the volume is low or high. It _only_ does it when the radio is not tuned to a station. When it is tuned to a station everything's fine.