Was digging a pond and heard an intermittent scraping kind of sound coming from the left track on my 931B. Used the loader bucket and hoe to lift that side of the machine. Held the right brake and put it in gear. Seems to turn fine and no noise. Put it in reverse at low speed and the track turned (made a noise) then stopped. Put it back in forward and the track turned fine with no noise. After a careful look saw the hub side of the idler was getting a bit chewed up by something. Further inspection revealed a rock wedged in-between the the idler brackets at the bottom rubbing on the idler. Couldn't figure out an easy way to remove it. Could get it loose where it would move but no way to pry it out. After a few forward/reverse cycles got lucky and part of the rock sheared off enough I could get it out. Not a lot of rocks where I'm digging but must have been a fluke I picked that one up and it got in behind the idler bracket. Glad it's out.
Sucks when they do that. Air chisel and a compressor about the only other way to alleviate.
Now I know what the pop noise I heard earlier was.
My D8K pushing to a crusher gets rocks caught between the frame and the sprocket. Makes a terrible screech. Be cautious when pulling them out because it doesn't take them very long to get hot enough to burn the skin on your fingers.
Rock wasn't hot but had a metal looking appearance on one edge from rubbing on the idler. Kind of a dull gray color like someone was hammering on it for a while. The rock was more of a white color.