As above, if the cambelt has snapped/stripped, the valves contact with the piston crown, and the force travels down the conrods, and can damage the big end shells, so in most cases should be replaced.
Damaged big ends can cause a tappy sort of noise, which sound like it's coming from top-end, but closer listening will reveal its lower down.
they can be replaced with engine in car.
this does not mean, of course, that this is what has happened to yours, and could also be just a tappet that hasn't filled with oil properly, or is slightly sticky.
re-built engine should always be turned over without spark plugs initially to get the oil circulated
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