I have a 99 4runner with 165k miles. The problem I am having is on three occasions I have heard a knocking noise coming from the engine. Two times were after driving the vehicle on the highway for around 2hrs and the third time was when I got home from work which is about a 20min drive (No highway). I took the vehicle to a couple of mechanics to try and recreate the knocking noise but they were unable to recreate the issue. I have also tried to recreate the knocking noise without any luck. I have never seen the CEL come on so I have no code to help diagnose the issue. The small dealer I purchased the vehicle from (I've owned the vehicle for almost a month) is willing to help me out but since two mechanics have been unable to recreate the issue there is nothing currently to fix. I was able to record audio when the knocking occurred and both mechanics fell like the sound is from valve slap or possibly a rod.

I guess my question is, what could cause a knock to come and go so infrequently? My first thought is low oil pressure but it seems like the knock would occur more frequently if the oil pump was going bad or a valve was sticking.

Thanks in advance for any advice you might have.