My 931B is working good but I was loading some clay I dug with the backhoe and sometimes(not always) the left track made a squeaking noise like something was caught in it when going in reverse. I picked the track off the ground and put it in gear and everything seems fine. There was no dirt or anything packed around the sprocket so I'm wondering what would cause that? I used it for few hours yesterday and didn't have any problems other than the ground being super hard. I didn't check at the bottom of the sprocket if there may have a been a little rock something stuck but it did look like the track wasn't centered on the sprocket.
Sounds like your idler might have collapsed.
When you go into reverse the idler pulls back into the recoil spring, could be nothing more than the idler moving back into the track frame a bit, or as Cmark sez, either the idler or recoil is in need of repair.
The undercarriage and idler only has about 1200 hours on it.
When I go back out, I'll check the track sag and if maybe there's a little rock or something in the idler area. It looked like the track was pushed over a bit instead of running in the center of the sprocket. I think that track might be just a little loose. I've read it's better to have tracks a little loose and than too tight but will a loose track cause alignment problems and/or a squeaking turning sound intermittently in reverse? It's not real loose like it's going to come off but a little loose.
The tension of the track shouldn't make any difference to its alignment. You should probably check your carrier roller as well.