Classic/Antique Car Repair: 350 chevy, 350 chevy, chevy 350

Question
Have a chevy 350 of unknown background. Probably from the 70s and some work done as  numbers .040 were stamped on front pad.
Drove car w/engine in ran fine then purchased engine.
Stored and put  engine in different car 4-5 years later and after it warms up I get a sound like somebody left a screwdriver in the oil pan. Deep intermittent rattling noise.
When spark is pulled from #2 it sounds like somebody knocking on the door.Coincides with and consistent with timing light.
Other than that it runs smooth. Time for the machine shop?
Or could I pull pan and check/change bearings?
All accessories are basically new.
Thanks for your time, Art

Answer
Is the engine skipping? It is possible that it has broken a valve spring or some other valve train problem. But that would almost always be associated with a skip. It might be wise to pull the valve covers first. If the upper end is ok it sure indicates that some internal service is needed. If it was my job I would have the engine out of the chassis and on a stand. Then pull the pan and check the bearings both visually and using plasti-gage. If the bearings check out then pull the heads or send it to the machine shop. The .040 means that it is probably bored to forty thousandths oversize or the shaft is forty under. On an older engine of unknown  history.
Is this car automatic transmission? If so it could be a loose or broken flex plate. The flex pate is the light flywheel that attaches the torque converter to the engines crankshaft. If is is just loose bolts it can most likely be tightened without removing the engine. If it is broken either the engine or transmission must be removed.