As the title says, there is a really loud knocking/slapping/clacking sound coming from the engine. Oil pressure, when warm is around 40 but occasionally jumps to 80 in the morning when the engine is still cold. Any idea what it could be?

 

Does it go away within a few seconds after startup or is a constant sound?

 

I only left the truck on for about 1 min, but no, it did not go away

 

Possible spun bearing, possible balancer shaft bearings, piston slap, the list goes on, More info would help.

 

I will post some more information tomorrow, from what I could tell, it sounded like it was coming from the top of the engine -- I'm thinking that it could be a collapsed lifter? What exactly is piston slap?

 

Well, I went to start the truck today. The knocking was present but went away after I revved the engine. Strange, but I'm not complaining.

 

I had the exact same thing happen to my truck. It was right after an oil change so i just assumed that a lifter got stuck and unstuck after revving it. I didn't ask any questions either lol

 

try to get video and upload it, if its coming from the top of the engine, and then goes away, I would say its a lifter. start it, and just let it run, and see how long it takes for it to go away
I ocationally have a tick in the top end of my engine (lifter) when its really cold, but once the truck is running for about 20 seconds, the lifter gets filled and is fine, but this is only when its really cold, try to get some vid.

 

possible issue could be incorrect oil filter. i currently work for nissan and worked for honda chevy and ford, i went where the wind took me by wind i mean money. ive always owned a chevy or gm product though.we see this issue a lot lot with people who decide to use other than factory filters. By using a other than factory oil filter back flow can be compromised casuing a temporary knocking like you've described. ive used wix, master pro, fram and experienced this issue but when i use a/c delco filter, issue is resolved. hopefully its that simple for you, if not hopefully you figure it out hpe this helped.