Prusa i3 MK2 z calibration problem (setting nozzle & probe height)

I purchased this Prusa i3 MK2 assembled. At first the Z calibration went fine.
But now when it is 2 days old, during Z Calibration the hotend is hitting the bed and making a chattering sound. That throws off the Z axis (forces the right side up). Even after leveling the Z axis this recurs. And the PINDA sensor appears to be centering on both the first and 9th circles which is what it is supposed to do. The PINDA sensor is also upright and at the same height it was when it arrived and worked.
Hopefully I can write to Prusa and get some advice on what to do.

Видео Prusa i3 MK2 z calibration problem (setting nozzle & probe height) канала maximpulse