Ah, then yes it could well be a ground loop.

You can eliminate the possibility of a ground loop by doing the thing you said you wanted to try: unplugging the audio out cable and switching to FM. The sound quality may be crap (or it may be decent, I dunno), but if the specific buzzing noise goes away when you unplug all cables and switch to FM, then you know it's a ground loop of some kind.

If the problem remains when you switch to FM, then the problem is either the Android inserting noise into the Bluetooth data stream (unlikely), or the circuitry on the Mini itself sucks and is inserting unnecessary noise into its output stage.

Just to be sure: The noise isn't there at all when the car stereo is just playing a CD, right?