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in bottom of an old 350 engine, for a few seconds right after startup, then goes away, fine otherwise, not horrible but its there, sounds like in front of engine low,
Yes,It could be a loose main or rod bearing--I have had several motors that knocked upon starting,usually worse after sitting or when its cold out(cold thick oil pumps slowly,you arent using 20w50 in the cold weather are you??) and it takes the oil a while to pump up to the bearings--on fords or other engines with the oil filter mounted on its side,using a cheap filter without an anti-drain back valve will let the oil drain out of the filter and back into the pan,this can cause that "woodpecker" noise when first started too,until the oil fill up the filter again and gets to the lifters and bearings..but since chevy'sV8's have the filter mounted vetically,it doesnt apply to them...
The motors I had that made that noise lasted as long as I had the trucks and beyond,I was just careful not to race the engine up high when it first started..I would not be overly concerned if it has decent oil pressure when hot,and you dont hear it any other time...but it could be an early warning of bearing failure too.so you might want to keep another motor handy,or rebuild yours,but the ones I had that knocked went a long time for me, and the guy I sold them too....