Ideas on Loud engine knocking, 2003 Elantra 2.0L
Kind of a long story, will try to summarize here. Car is a 2003 Elantra, 4 door, pretty vanilla. About 180k miles. Was running great until my son overheated it and warped the head. I pulled the head, confirmed it was warped, took it to a shop and had it fully rebuilt. Reinstalled the head, running great again. Everything was fine for about 1500 miles, at which point it started making noise that sounded like valve lash. The 03's have mechanical valves that can require periodic adjustment, so i figured the shop didnt get the lash right or something, and planned to pull the valve cover over the weekend and check. However, a couple days later the noise got worse, a loud knocking. I figured the nead was screwed up, either valve hitting a piston like timing issue or something in the bottom half of the motor let loose. That kind of loud. Got it towed home and decided to pull the head to see if there was anything obvious.
Pulled valve cover and spot checked some of the valve lash. Was all fine. Couldnt see any broken springs or obviious problems, and when i started it up with the valve cover off everything was functioning as it should, besides the loud knocking sound. Sounds like its coming from the center of the engine, so still thinking valve hitting piston or something.
Got it all torn down and head off, everything looks fine from a visual standpoint. No evidence of valves hitting pistons, no damage to valves, piston surfaces look fine. Bottom half of engine spins nice, no noise.
At this point I'm stumped. If i threw a bearing or something on the bottom half, would it make noise when idling but not make any noise with the head off and engine spinning? Could there be something going on in the head that isn't obvious from a visual inspection? Any tips on further troubleshooting? I'm considering throwing the head back on and spinning up the motor to see if it makes the knocking noise again, but running out of ideas. Close to just having it hauled off, but i'd really like to get it running again for my next teenager to drive rather than buy something.