Well that was a first for me. Jenn stopped me outside the house getting home from work and said the van was making a weird noise. We rode around the block and heard a squeal from the back right wheel.
I looked underneath but there was no sign of something jammed under there. I jacked it up, and sure enough, manually turning the wheel elicited a grinding squeal.
I took off the wheel, and damned if there wasn't a rock stuck between the back of the disc and the disc rock guard (the irony). The gap is only a few millimetres and I couldn't get it out.
After a minute of contemplating removing the caliper and disc, I figured I'd try to floss it out like a bit of beef jerky between teeth. I used a nylon tie-down strap between there like my back molars, and pop! Out comes a little rock.