97 xj w/ 152k on cold start sounds good but after a couple of minutes you can hear a loud knocking that sounds like its coming from bottom rear of engine. After its warmed up it goes away I thought it was the dreaded flexplate knock but I've checked the flex plate bolts and they are fine and it doesn't appear to be cracked. Not sure what it is but its annoying and it does it everytime you start it up then goes away after a few minutes of driving. Any answers would be greatly appreciated

 

Could be piston slap. I heard for alot of 4.0l this is a common problem. But usurally you hear it from start up until the motor is warm. The pistons expands from the heat, taking up excess space between the piston and cylinder wall. Once this happens the sound goes away. Im not sure if anybody really does anything to fix it. All I have heard is people just let them warm up before they drive because the sound is annoying while driving. If I am wrong please someone correct me so I can get my facts strait.

 

I've heard this too.

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Hey you got a 4.0, they make some noise, but they will make that noise for hundreds of thousands of miles with no problems.

 

i had same issue and still ended up being the cracked flexplate. I went as far as dropping pan and looking at bearings. I continued to let it make noise and drive it and it got worse and didn't go away when hot and then checked flexplate again and was cracked all over. If the sound changes when you put it in gear it is most likely flexplate or crankshaft end play. If it doesn't change but still goes away when warm the piston may be the issue.

 

A pic of the noisy beast,

Piston slap it could be I guess, it really is annoying though cause it sounds like the engines blown up or something then it just disappears. It's kind of a loud slap ticking sound if that makes since, when it first started it almost sounded like a diesel engine with a cold lol!

The sound is defiantly on the bottom end toward the flexplate area but then again it sometimes sounds like it's coming from the front of engine. Hmmm

 

The fact that you mention it sounds like a diesel reminds me of mine. Just rebuilt it again recently as a stroker and as of 2 days ago it has a much more pronounced diesel'ish tone to it which subsides after a few minutes.

Ever since I built it though it always had a slight diesel sound to it...

but when I rebuilt it the number 6 cylinder had to be rehoned to clear out some metal that was scored into the wall from the piston when it overheated. The machinist said that cylinder would burn a little oil from then on.

..the sound has gotten too diesel'ish for my liking... yet after 3 rebuilds it's time to let it lie until something catastrophic happens. .. .. Again.