I have not noticed any grinding but TBH it was running unattended for a large part of the run.

Both times it has happened the drift is on the X-axis. I might try to re-print a smaller version of it orientated a different way and see if it still drifts along X.

I am not sure how tight the belts should be but they certainly don't seem loose. There is very little give on each. Most give is on Y axis but there's not much in it.

Thanks for the tips. I will try another print tonight and see what happens.