Jeep Engine Noise Progression
 

I bought my '96 XJ 4.0 auto in August last year, 175,000 miles. The engine was fine then, no strange noises. Last month I changed the oil will 5w-30 full synthetic and that is when I first noticed the tick. I figured it was just piston slap from the cold. I have been paying close attention to it and I realized at warmer temperatures it does quiet down, but it is still noticeable.

I also realized I am getting 12mpg which is I am really not happy with considering I only have put on 31" tires and I don't think that will cause that. This made me inspect the exhaust system. I found a manifold crack.

I have four suspicions for the noise:
-Manifold leak
-Thin oil
-Lifter tick
-Piston slap

I ordered an APN manifold and it recently came so I am planning on putting that on sometime soon (I need to get a flex joint to prevent another crack, and that will go on with my new cat because mine sounds like a can of nuts). I will find out if that affects it.

If not, I will move on, flush the crankcase, and change the oil to a thicker and traditional oil because I feel the thin synthetic might being making the engine unhappy. I've also suspected that the synthetic with all the detergents might have clogged a small oil passage and is causing the noise.

If neither of those are the issue I will check my lifters and if not those I will assume is it the piston slap or some other issue and will diagnose it from there.

I will post my progress for anyone who has the issue or is just curious. I can also post a video of the noise if anyone wants it.

4.0's can be a noisy motor. Some find certain brands/types of oil are quieter than others. Synthetics seemed noisier to me, YMMV.
You have a 1996- the factory cat is under lifetime warranty. Give the dealership a call and see if you are eligible for a free replacement.

I won't be putting synthetic in again, just because of the cost over traditional oil. And I've heard about this, but I wasn't sure whether it was true or not. I will definitely give them a call. Thank you.

Here's info on the Cat recall if you need it:
http://www.jeep.com/webselfservice/pdf/E22.pdf

Full synthetic is not bad for your engine. It is not any thinner.
You could be hearing your mani/or exhaust leak. They sound like ticks sometimes.
Fix your mani & install a good gasket with new bolts.
Then give an update.

You will see a decrease in mpg's with the 31" tires over stock as well you have 3.55 and a 4.10 is called for using a 31.