I would guess that the tooth-grinding is contractions. We too look for the hollow in front of the pelvis that indicates the lamb(s) have moved into birthing position, but it's not always that obviousn in some breeds, particularly if the lamb is malpresented. If a ewe's been on her side with her legs stretched out, pushing, for more than 20 minutes and no sign of nose and/or feet, or the nose and front feet are there but not progressing out, I generally scrub up and investigate, just to make sure the lamb is lined up OK, it's not a big one that needs a pull and the ewe isn't suffering from ringwomb