My 4.0 does same thing its quite for about 10 seconds when first started then makes that same noise . Have yet to have it checked out. Still seems to run strong. It does worry me though. Just curious if anyone has been quoted for a rebuilt. A mechanic wants my old car for his step son and offered to fix up my jeep but i dont know if i asking him to rebuild my engine would cost more then my cars worth. Which is around $1000

 

What brand oil filter are you using?

 

Haven't had it for long but previous owner put a Wix oil filter on it but it looks tiny. Its not the 51515 some other number i forget.

 

51085 or 21085?

 

TwoXJs, that sound definitely could be a broken skirt however, if it were broken you would have found the pieces in your oil pan... they don't just disappear. This is what I found in my buddies oil pan (2.5L JK)



As far as a rebuild, the pistons are made of aluminum and the block is iron, provided the bore hasn't been scored by the piston you could probably just replace the piston with a new one which runs about $40 plus all the ancillaries like head, valve, intake, and thermo housing gaskets. It can be done without pulling the engine. I hate to say this but I would pull the pan again and check the underside of each piston for missing bits before you make a decision. Pull the plugs and have someone rotate the crank while you watch everything go through the motions.