It sounds and feels like a single hammer hit to the frame just under the seat. I can't seem to figure out what sequence causes it, but it has happened a few times. It's not the Centra compensator drive, nor is it the clutch.
Thoughts?
It's the feel part that you describe that concerns me. You also mention it's occasional.
I'd take the engine out of the equation by putting it in gear with the engine off & yanking it backwards several times trying to emulate the condition.
Backlash in the transmission wouldn't be felt like hammer blow, so I'd be looking at running gear colliding with frame somehow, and that's a visual check all over.
It's an unusual issue you describe, so aim for a methodical diagnosis & let us all know when you find it..!
