Spun bearing(s) or failed oil pump?

Supercharged111 said:

The pickup should be welded to the pump or else they sometimes fall out.

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I've never seen that. I've always heard they are pressed in.

It’s what melling recommends after installing the pickup.

How many smallblocks have you built? The pickup tube is pressed, to have positive attachment one or two Tig tack welds. If your worried about big end rod side clearence check the Unit repair section engine assembling. The rods don't knock against each other like your thinking. There cycling up and down, wrecked or damaged bearings causing excessive clearence, resulting metal to banging when loading direction changes.Rotational bearing, up, stop, down, stop. Pistons are made of different alloy, rods crank block, heavy iron. Get rid of poppet F I system. Get upgrade electric injection. Delphi on my 98 Tahoe way better.

There's a spec for side clearance on the rods, and you'll likely never find a motor that's in spec. It doesn't seem to be a problem. All mine had "too much" clearance.

The best way to confirm or eliminate a bearing issue is to pull each bearing cap off and inspect the bearing and journal. A failed, "run", or stacked bearing will be pretty obvious. With an engine that small, you should notice any vertical or side to side (with respect to the crank centerline) movement in the rod when trying to move it by hand. Main bearing is also possible, but those are usually deeper in tone and reasonably quiet at idle.

The "thin metal or plastic" looks similar to what we call "shim stock" that happens when a bearing is pounded out of a connecting rod. If it is attracted to a magnet, that could be your issue. Again, bearing inspection would give a definitive answer.

I have also seen/heard lifters that lose prime make a pretty bad "knock" if they stay that way long enough.