Lately I've been hearing a snapping/ knocking noise on my (07) L3400 DT with 600+ hrs. Sounds like it's coming from the left rear side, but it's not constant, so it's hard to pin point the noise, it sounds like a hammer tapping on the wheel when it does it. I don't know if the diff. lock is making that noise or not or even if a manual transmission will make that kind of sound, if it could, why doesn't it make that snapping sound steady? I have noticed that the diff. lock petal will squash down 1/4" more at times, and yes I have checked the lug nuts.