So my dad got his BMW M6 like last November, and ever since we got it, everytime we unlock the car, it makes this weird sound, that just continues for like 3-4 seconds, I'll try to get a clip of it, and whenever we're driving around, if you've ever played Halo, Halo 2, or Halo 3, whenever you respawn, it makes those three beeps, well that's what it sounds like, just that random sound comes up, exactly same, maybe different tone, and we've unable to sort it out.

If you could please answer my question, I would appreciate it.

Is it a whining sound? If he has an SMG, it's the pump priming the system.