A small rattle?
A few things I can guess at with little chance of being right.
1. We've seen a mod here on the forum where the crank was modified for, I think, 180 degree alternating up and down strokes between bores. I doubt that's what's going on here, so I'm not going looking for that topic to expand on the details.
2. Little end broke right next to the gudgeon pin letting one piston stay up while the rest of the rod rides up and down in the bore and, at the same time, the other cylinder keeps running to propel the bike. (Talking to self: "But Richard, wouldn't the broken rod ultimately push the piston at least to the top of its stroke?")
3. Piston fractured circumferentially at one of the ring grooves with same result (and same talking to self).
4. Broken crank?
If a rod cap came off, the devastation to the crankcase would not be subtle.
I should just stop because, in fact, I really don't get this if the engine was still capable of running before disassembly.
Very anxious to see the truth.
Richard L.