I just finished testing him the same thing Puma. I actually had the same thing happen on my motor when I first got it. I could grab the flywheel and move the crank up and down. Figured it was bearings, however after disassembly I found the bearings appeared to be good. I could however drop the bearing in and out of the cases. Ended up replaces both case halves.
I can see a few things causing this issue such as detonation hammering on the bearings. Or even bearings going bad which causes them to beat the cases. Improper installation or extraction of the bearings or multiple rebuilds just wearing out the bearings.