All had chrome bores. If it rattles at idle, it's possible it seized at some time and scored a piston, but was freed up and continued running (I've done that, they do get noisy). A compression check would probably show that. However, it could be that something is loose, like the crankshaft gear or clutch nut. There are other possibilities further into the engine, like rod or wrist pin bearings. Those are highly unlikely unless it is very high mileage or got rusty internally sometime in the past. Someone else may have better suggestions.
