Hello,
Just bought my first 4Runner, a 1998 4 Cylinder 5 speed with 220k miles from the original owner. Got a deal on it (at least I think so..) because it has what the seller described as "top end noise."
Here's what I'm experiencing:
When first started, runs pretty smoothly with a subtle *clack clack clack*. I hoped when I test drive that this was valve noise, knowing I can adjust those myself.
However, once the engine is good and warm - maybe 10-15 minutes down the road, this noise gets louder. When I pull over and listen under the hood, the noise sounds more like a knock..

The second issue that seems to go hand in hand with the noise is lack of power. I have read enough around this forum to know not to expect much pull out of the little 2.7 4 cylinder, but there definitely seems to be an issue. In first gear it took everything it had to get up the small hill in my driveway. The Honda Civic climbs that just fine. On the highway I have to downshift into 3rd to make it up gentle grades doing 55 mph.

So far I've been told by friends it might be (please no) rod knock, a timing issue with spark not advancing, or an oil pump causing other problems.

Doubting it's anything catastrophic bc of good maintenance records and relatively low mileage.

Any similar experiences? Ideas for diagnostics I can run?

Sounds like a sticking valve.

Do a compression test, cold and hot.

Just did cold right now: all cylinders hit 125. I'll check hot once the rain stops.

Also noticed that the oil light stays on below 1000 rpm, and flickers/shuts off at rpm's above 1000.

Is an oil pressure check in order? Also the internet tells me a bad knock sensor could give me these knocking/low power symptoms. Planning to check that with the multimeter soon.

Exhaust leaks can make a ticking sound.

One of my friends had just a bought a 1994 Miata 1.6L 4 banger, and he described something very similar. It turned out, he had an exhaust leak and his exhaust pipe was rattling. Easy fix but for some reason there was still a knocking in his engine bay, although it was more quiet after the exhaust fix. He ended up just adding a quart of oil and bam, it was fixed. I would assume your pistons and such aren’t being lubricated enough maybe due to lack of oil and oil pressure. If you remove oil from the equation, your car can make some weird noises!

I don’t think a quart of oil is going to solve your problem, although I would suggest doing some heavy maintenance to remove any outside factors. An oil change, check the fluids and brakes, even the rotors. I hope all goes well for you! I’ve also never seen a 4 cyl. 4Runner so I’m a little shocked here haha!