Bevel drive bearing failure
I was in the garage doing some maintenance on one of my bikes this afternoon, so in a quiet few minutes I took the bevel drive apart to see what the problem was. As predicted, it was the bearing nearest the wheel that had started to go. You can clearly hear how bad it has become in this video.
Next steps are to get the shaft pressed out of the assembly, remove the failed bearing, refill it with oil and reassemble it as a back up.