It's probably not serious but you should fix it at some point.
There are 4 rods in the outer edges around the top of the machine. I think one of them is sliding back and forth a few mm. The next time it is printing put your fingers on the ends of each of the 4 rods and see if they move a little bit. Once you identify the loose one memorize which of the 4 rods. Later when nothing is happening you need a 2mm hex wrench (don't use inch measurements or you could strip it) preferably a 2mm hex wrench with a screwdriver handle but the L shaped wrenches are fine. Also the ones with the ball end are even better.
Push the rod both ways and decide which way is better. On the side that is now loose you want to loosen the pulley that the long belt goes over - then slide that outward against the side of the printer and retighten - tighten the hell out of that one - it was obviously not tight enough the first time. The wrench should twist a bit with the force.
Here is a video with the described noise:
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