On mine I could make it go off by driving over a bump or somtetimes just shutting the tailgate. Leave the radio on and try pulling the unit or giving it a thump. If the sound goes then you know you are at the correct place.

FYI: BMW said on mine it could be the steering wheel and they wanted to replace that first, then charge me for a new wheel even if that was not the cause. But I noticed that the sound almost always went off after some vibration and most certainly if I slammed the tailgate shut.