94 4Runner 3.0 with loud engine knock

I just purchase this 4Runner. It ran great for around 500 miles. Now it has a rather loud engine knock. It sounds like it is coming from the drivers's side around the valve cover. I did remove the valve cover but did not find anything wrong. It will completely quit making the noise some times and run just fine. It may knock for 20 seconds and then just run quite for 30 seconds then start knocking again.
Has anyone experienced anything like this, and if so do you have any ideas?
Thanks in advance for the information

Sounds like you have a serious problem with oil supply to a rod bearing or wrist pin. Could also be piston slap. Hard to tell without hearing it. I assume you have checked the oil level. It may be as simple as a new oil pump.

A temporary fix might be to run a higher viscosity oil, like a 10W40 instead of the recommended weight. I bet whoever sold you the truck has a higher vis oil in it already as a "fix" just to quiet it down long enough to sell it.

You should be able to tell which cylinder it is by pulling the plug wires one at a time. The wire that quiets the engine the most is the bad cylinder. Compression check may show the bad cylinder if it is piston-related.

None of these possibilities are good. Take it to a mechanic you trust and hope for the best.

I hope someone else here can provide a more happy guess about this problem. I am just guessing without hearing the enginer run.

I just have never heard one quite back down to normal if it is a problem like this one. Usually if it is a rod or piston the noise stays constant, but this will run normal at times.
I read in one forum that a guy had a problem with a water pump and it had a noise similar to this one. Any ideas on that?
Thx

Also, I tried pulling the plug wires one at a time, but nothing changed the knocking sound. I figured that when I pulled the problem cylinder the noise would stop. It kept going no matter what I did.

It's tough when you can't hear it. I'm out of guesses. Get a stethescope and see if you can locate it. Piston slap may not change much by pulling plug wire.

I have never heard a water pump knock.