In my case I looked at the mechanism and saw no immediate failures

I then noticed that a black strap (Arrowed) was showing signs of being curled/crumbled

Further examination shows that the strap, which goes from top to bottom, was not aligned 100 % centered between the massage wheels (I try to show that with the red line) and the strap had become a little loose

The result was the that the right wheel was sort of "picking up" the strap, so both wheels was on the same side of the strap

At some point the right massage wheel had to cross back again over the strap hereby crumbling it and giving this jerky bump.