Join Date: Jan 2019
Location: Eagle Lake, FL
Year/Make/Model: 2008 en500c
Before I go any further, with this new information... My advice is to NOT ride the bike until we determine the cause of this noise.
Since the bike was in neutral when you were spinning the tire, it's most likely not in the engine itself. It is beginning to sound like a bearing issue inside the transmission. Not 100% sure on that yet, but it's beginning to look that way. A trans bearing locking up or giving out could very likely lock your rear wheel up. You can imagine the rest. Another possibility is a rear wheel bearing, again, not good if it locks up.
Since I wear both hats at various times....
Shop mechanic says absolutely no riding the bike until the cause of the noise is determined and repaired.
Home mechanic says no riding the bike more than up and down the block for troubleshooting purposes, listening to noises, etc. But be ready in case something gives out and causes a major issue.
Next step mechanically is to isolate the engine/trans from the rest of the driveline. Basically, take off the belt/chain. Check the rear wheel alone, then turn the drive sprocket. If you still have noise at one or the other, You're getting closer to the cause. Also try shaking the drive sprocket up and down and in and out. Basically, shake, wiggle, push, pull any piece designed to move in every way possible. Let me know about any play on the pieces, especially areas like the drive sprocket or rear wheel.
I wish I had better news, but there is still hope. A trans repair is definitely something that can be done at home if you have the patience, time, and place to do it.
One other question, do you have a repair manual for the bike? If not, get one. You can usually download them free from manualslib in PDF format.