I had forgotten about this post, what with all the parrot business. But I've noticed some grinding sounds in the past few days.
For me, it mostly happens during fast circular moves - the insets before my slower perimeters, when I'm feeding at 80 mm/s. Oddly enough, it only happens in one direction (clockwise, not counterclockwise). So I'm pretty sure it's not a hardware problem. The first few times, I thought I was extruding too much plastic and I was grinding against the previous layer. But I've checked it many times, and that's not the case.
The only explanation I have is that there's something weird with the slicer settings or the firmware. These prints were generated with MiracleGrue at 0.10mm layers.
I can live with it for now, but it's slowly starting to get on my nerves. Primarily because the sound always makes me worry that the print is failing.