"I recently purchased a 94 4Runner. I have only ran it for about 2 weeks and 500 miles. It was running just fine when it started to have a really loud knocking noise in the motor. The noise sounds like it is coming from the driver's side, at the top of the engine. This is what I think, no way of knowing for sure. I pulled off the driver's side valve cover because it sounded like a spring was broken or something. Everything here looked just fine.
The odd thing is that the noise is not there all the time. Sometimes it will run for 30 to 45 seconds and sound normal, then this knocking comes back."

More Information:
Well, I pulled off the oil pan and after awhile found that one of the rear rod bearings is rattling pretty badly. I was going to just put in a new crankshaft till I found that they are over $900.00. I am trying to decide what to do to fix the problem. I hate to pull the engine, but it looks like that is the next thing to do. What are the chances of cleaning up the crank with emory cloth and installing new bearings and having it hold?

Thanks

 

Could you take the crank to a machine shop and have it slightly undersized and then go with a bigger bearing if it is damaged that bad?

 

Look at "934rnr"s post in this thread:
https://www.yotatech.com/forums/f2/clackity-clackity-clackity-34351/

He explanes doing just that, provided the crank is not fouled.