2005 5.3 Vortec Sounds Like A Harley

I would start by picking up a mechanical oil pressure gauge so you can see just what is going on with the oil pump. May also want to do an exhaust vacuum test, look for leaks and/or restrictions.

Thanks! I'll do that. Any advice on where to install the gauge? I'd rather not try to permanently install it through the firewall. I know there's a small protrusion above the filter on the pan, but I can't remember if it just bolts on and can be removed and drilled/tapped. Is there a better place to just tee off for the gauge?

The gauge screw into the same spot as the current sending unit, and cost around 35~50. This video may help, same concept works on any car/truck. You don't have to touch the firewall, just connect the gauge and if possible, position it so you can see it through the windshield, as it's only on there temporarily. This will let you know if you indeed have low oil pressure. Video title (How To Check Oil Pressure -EricTheCarGuy): https://www.youtube.com/watch?v=I8teB16U0LA

Just reread your question, just so we're on the same page, I meant install a temporary oil pressure gauge, not a permanent mechanical gauge for the dash.

Oh cool, just put it in place of the sending unit. I'm probably going to just fabricate my own adapter for whatever threads the sending unit has, I've got plenty of gauges I can use. Stay tuned: As soon as I have some time I'm going to get it back up on the lift while it's running and figure out exactly where the noise is coming from. Too hard to tell when I'm listening from the cab or from outside the perimeter of the car.

Well, just to wrap this up (hopefully): I took the oil pan off again today for another look and to check the oil pickup tube. I got the idea that the o-ring on the pickup tube was the culprit, so I replaced it with a new one. I was very careful to make sure it seated correctly when I reinstalled it. When I started the truck back up my oil pressure jumped up past 40 and looks perfect again. One problem down. But... When I had the pan off and I was checking the crank out hoping to see an obvious problem, I happened to notice that my flex plate is cracked in 3-4 places. It must have been turned just right to hide them all the first time I had it opened up, and they're not visible from the inspection hole on the transmission. I turned the crank a few times and saw the plate visibly rattling against itself where it's cracked, and I heard the same clunky sound I can hear when it's driving. Seems to explain everything I've experienced, so I think I've identified the problem. Even with a 2-post lift and my old man's shop, it's not a project I can get done before I have to go back to work on Monday, so I'll just limp it into a shop on Monday morning.