Yesterday I was driving around, and the nock nock sound coming from the right side of the car was driving me nuts. I stopped on the shoulder of the road, leaned to the right and started bumping stuff to try to repeat the sound. As I leaned back to my seat, I pushed the passenger's seat and "nock", it made the sound again. Pushed it again a couple of times and it repeated the sound.

Turns out the passenger seat belt was locked in between the seat and center pillar, making noises on each bump:



After I moved it parallel to the pillar, the noises went away:



So simple, yet so lame.