I found this website and was checking out the sounds:

John Muir's Engine Sounds and more...
http://home.arcor.de/silke.fokken/sounds/

The sound on this line:

Loose Flywheel (115 KB) - similar to a broken crankshaft. Either way,
pull the engine. Odds are you're looking at crankshaft damage
anyway...meaning overhaul time.

Actually sounds a bit like my sound. Also, I did have trouble with the
clutch/flywheel as I mentioned. So I'm putting fairly high odds on
this being it. Hopefully it's not too late to correct. I may find
buggered up dowels and alignment holes if that's it. Joy. I torqued
it as good as I could muster, but I could have gotten it wrong.

I'm still looking for feedback if you have any. This is probably it
but may not be.

Chronologic