The final conclusion I made was that I had broken some glass in it a
month back and there was still some glass in there somewhere banging
around. This was confusing, since we had only been recently hearing
the noises and the sound was not coming from the tub, but from
underneath the tub... so, I took out the bottom basket, unscrewed the
filtering thing that catches large objects/food, and what do you
know.... sitting at the very bottom of the hole where the water gets
pumped was not glass, but a cherry pit!

This makes perfect sense since a couple weeks ago I had eaten a bunch
of cherries and threw all the pits on a plate. When I scraped the
plate, I must have missed a pit that got stuck to the plate, and into
the dishwasher it went. the pit was small enough to slip through the
filtering grate at the bottom of the dishwasher and, as the water is
getting pumped through the dishwasher, it's perhaps hitting the pumps
propellers (?) causing this grinding sound.

Good sleuthing!