RE:
I just got my COREOne Kit built. I had the 1-2 minutes of head banging BECAUSE IF HAD FORGOTTEN TO TENSION THE BELTS.
If you LOOK at the menu while the banging is going on, you will see that the ZEROING step is failing to ZERO ! So the printer keeps repeating the 2 taps on X and 2 taps on Y to try to get a successful home.
The troubleshooting instructions state that ONE of the causes of this problem is that the belts are too loose.
I suppose what is happening is that the belts are so loose that the motors do not stall out, the belts just slip.
From multiple threads on this topic, it seems that the more common cause is not the belts being loose, but the X gantry not being orthogonal to the Y axis. The bump against the front end-stops then becomes a double-bump (left & right end stop at slightly different times), and apparently the firmware cannot derive a precise reference position from that.
The observed behavior is also different in this case from what you describe: It's the diagonal bumps which get repeated endlessly, not the right, right; front, front double-bumps.
Edit: Prusa's new guide on belt tuning explains how to obtain an alignment which avoids this problem.