Issues with first layer calibration.  

RE: Issues with first layer calibration.

This is normal behavior if I recall correctly. I suspect you mean the same behavior where if you press "Load to Nozzle" and start a print you'd get the same ugly grinding noise. Both scenarios are eliminated and much improved with the next firmware release (3.13.0) which will hopefully drop very soon. The MMU has been the biggest focus for that release.

I'm sorry I can't help more, just thought I mention there is a newer firmware coming out very soon.

RE: Issues with first layer calibration.

thanks for letting me know! I did notice that the newest firmware did have an upgrade for the first layer calibration, but now I hope that the new update will fix  the issue.

RE: Issues with first layer calibration.

That's definitely not normal behavior. Loading to nozzle and the first layer calibration works without any grinding noise whatsoever.

The grinding noise is most likely caused by the MMU2 not disengaging and still pushing while the extruder is already engaged and pulling. This happens, when the filament sensor in the extruder does not trigger correctly. Check: https://help.prusa3d.com/article/ir-filament-sensor-calibration-mmu2s_2245

It is essential for MMU2 operation that the two sensors (IR in extruder and FINDA at MMU2) are calibrated correctly and give constant signals if triggered. Sensor calibration is for MMU2 what first layer calibration is for the MK3: a bit difficult at first, pretty much the cause of any initial problem, needs to be done correctly and with precision, more often than not it's not done correctly at first try...