Are you runnig any other loads on the circut, like stereo or electronics that are not factory from BMW? If so when did it get installed? Check the voltage out put on the system. If you don't already know how to do it, here's how

take a multimeter and test the battery should be around 12.85- 14.00 volts for a good battery, if your less than that then you have a problem. (get a new battery) start the car and monitor the voltage meter which is attached to your battery. the voltage should go up when car in running. put your headlights and heater fan to medium speed and obsereve the voltage drop. but when this humming comes into play up and down watch to see if voltage of system follows. If it does then the might be a problem with the alternator. I would take it back to where it came from. If it doesn't (which I am certian it will!) then take it to an indy mechanic you trust. Good luck!