I have considered that could be the issue, I just don't see how it would only make that noise at that RPM and never make noise otherwise. I think that is probably the worst case scenario at this point but really doubt it's that.Ok, so that still narrows it down a bit. It might not be the engagement itself, but it does seem to need to already be engaged to make the noise, which tells me that it's not in the engine, but in the Clutch/Transmission. If you haven't already, do the test with the front wheels in the air, and listen very closely around the clutch housing. This might be a hail mary, but you might have damaged the dampers on the clutch disk, causing them to vibrate at the low RPM, otherwise, there may be a loose bracket in the vicinity.