BMW: Sounds like the, mechanical shape, source of the sound

Question
Hi - My son just bought a 1989 535i -it seems in pretty good mechanical shape.  Our question, about 50% of the time, when he starts it (automatic) a very shrill sounds comes on - some kind of alarm (maybe the seatbelt?).  We have no idea if it ultimately goes off because we turn the car off and try again.  Sometimes, buckling in and removing the key and then trying again seems to help, but isn't foolproof.  Thoughts?  Could it be an aftermarket alarm of some kind?  Is there just a "shrill sound" fuse we can disable?

Answer
Kristin,
 People put all kinds of alarm systems in these cars, and it could be a number of things.  I would first search for the source of the sound.  There has to be a buzzer or speaker creating this noise, and it's possible to just unplug it.  So do some searching under the dash for the source.
 Good luck,
 Josh