Perhaps there's a thread about this already, but I wasn't sure what to search for to find it. My xj (98 cherokee sport 4.0 auto) makes a strange knocking noise which I haven't experienced on other vehicles before. Mainly when the engines cold and idling (usually after starting but before taking her out of park). It's a low sound - not a tick - more like a thump thump sound and it isn't constant. Kinda like a subtle "thump thump....thump thump thump thump... Thump... Thump thump" and once driving around and warming up, it stops. I can also feel a slight vibration accompanying it. Any thoughts, or similar experiences?

 

Two things to check:
Exhaust manifold, make sure the mounting bolts are tightened properly(mine where loose and I had a knock.
Flex plate , the bolts can be loose and cause a knock. The flexplate could also be cracked. This sounds more like the problem you have. It's easy to check. Time for some noggin pokin

 

I had a knocking on my 99 jeep too and it was the flex plate, but I could really hear mine when I turned off the jeep. It caused some major problems for me later.. Ended up having to replace the transmission. Don't just ignore it..

 

Which way should the flex plate turn?

 

So I checked everything out and the flex plate is not cracked and the bolts to the Tc are tight.

 

Well that's good news! Did you do a really thorough check? Reading other forums they can be really hard to see..I would say check it like it say's in this forum... Good luck with figuring it all out



https://www.cherokeeforum.com/f51/ch...xplate-172081/

https://www.cherokeeforum.com/f51/ho...ocking-125547/

and if its not the flex plate..great one less thing to worry about.

 

I used those two as reference actually haha. And my plate still looked brand new. Barely any grime or rust. Maybe it's a tranny mount or something else rattling with the engine vibration

 

See if your engine and trans mounts are worn or missing bolts. If so, the exhaust pipe will "thump" on the crossmember.

 

I'm thinking it might be that cruiser. I haven't gotten to take a look today, but I had some sort of loud violent vibration from what I assume to be the tailpipe on the crossmember or something else.

 

It's as common as belly buttons.........