What car? LCI with ATC500 transfer case - meaning xDrive?
First guess - worn actuator gear. Easy to remove and check and if it's worn, rotate it 180Deg. until you get a replacement - or just leave it like that.
If the gear is ok, it's likely to be the motor itself...
*edit* Here's a DIY video re. the gear - ignore the bit about removing the motor from the actuator, as this is not required to inspect or replace the gear.
https://www.youtube.com/watch?v=UR7V...ature=youtu.be
Thanks to
richardb for the video.
