I don't know. Mine would come and go too. I'd go half the dirty sanchezz trail and I thought "oh good, whatever it was is gone"... then ten minutes later the noise came back. Grit and rocks rubbing the rotor will give a crunch scrape sound. I'm only 99% sure it is grit/dirt on the rotor in your case. Could be I am wrong
if you were in the mud though it would be good to spray it down anyways.
edit: take the rotor and unit bearing off to check it all out. Maybe it will become obvious after you get into it.