Auto calibrate sometimes thumps the bed, is this normal?

I’ve run the calibrate height then auto-calibration a number of times and I’ve tried twice to print the keyring but it failed each time. I don’t know if this is a calibration problem. Sometimes in auto-calibration the nozzle goes down and it sounds as if it is thumping the bed rapidly a few times, then it carries on. Sometimes it finishes the calibration without any thumping at all. Each time it says Calibration OK, I save the settings and try to print.

I’ve tried tapping the bed at several points all round and near the middle, the red light flashes each time although when it was making the thumping noise I didn’t see the light flash.

I faced that issue a week ago. I now have a few “impacts” on my Buildtak. I ended up redoing the belts (using force to have them as strong as possible) and making sure all rods had been well connected. Lots of oil and PTFE… auto height and auto-calibration. And it worked. No impacts so far.