1998 jeep wrangler making knocking sound

I have a 1998 Jeep Wrangler 4.0. When you first start the Jeep it runs fine until a few minutes after it has reached its full operating temperature. When that happens it starts to knock very loudly. The knocking sound is audible until you reach about 35 or 40 miles an hour then you can’t really hear it anymore. It sounds like it’s coming from the engine compartment. If you pull over and put it in park it stops. If you put it back in gear it starts knocking again. I was thinking it’s probably not something in the engine being as it doesn’t happen when it’s in park. You can rev the engine up and it still doesn’t do it as long as you’re in park. A little background, the vehicle sat for about 5 years. We replaced a lot of smaller parts in the engine like distributor cap and the other basics to get it running. Does anyone have any idea what this problem could be. And another little side note. I’ve never actually heard of vehicle knocking before so the sound may be different. I appreciate any help. My family was excited to finish our project and now I dont want to drive it.

I’ve heard a cracked flex plate make that kind of noise.


Forgive my almost complete lack of Automotive knowledge. Would that make it happen when it’s warmed up and in gear? Or would it happen all the time? And while you’re at it where would I find the flexplate LOL.

It depends on where it’s cracked.

Steel expands when it gets hot. So a crack can become larger when the steel expands.

When in gear, there’s a torsional load applied to the flex plate. When in park there’s no torsional load.

Remove the access cover from the transmission to expose the flex plate. Have someone rotate the engine slowly by hand at the crank bolt while inspecting the flex plate.

And if you’re unlucky, you’ll find a crack.


Thank you very much I’ll definitely look into that. That’s got to be better and spun bearing.