If i come to a stop and idle i hear a rattling noise, once i accelerate it goes away, any ideas?

 

my rattle is the catalytic converter, im waiting to do the whole exhaust at once before i have it replaced.

 

Yeah i would say something is loose underneath like the Exhaust system. Just when the Jeep is moving the Engine runes a little smoother since the RPMs are up.

 

Can you tell approximately where it is coming from? Front, rear? Did it start suddenly or after a repair?
I had one that I chased for months until I had to replace my failing radiator. I found out a grommet was missing under one of the 2 dowels at the bottom of the radiator. Major rattling especially when starting (accelerating) then it would stop at higher speed.

 

My Cat rattles, I hit it with my 3lb sledgehammer, solve it for a little while before I replace it.

 

Something internal on my cat convertor used to rattle a while back. Lived with it for a long time until the rattle changed and decided it was time to get a new converter. Something probably had come loose and shifted in there. Thought it was better to go ahead and deal with the cc than burnt exhaust valves...

 

The heat shield around the cat tends to rattle when the spot welds rust away. I just drill a small hole through the flange and run a self-tapping screw through it. I just did it to mine for the second time. You can tap on the cat with the palm of your hand to see if it rattles, I wouldn't suggest you use a BFH as you might break the element inside.

 

My cat has a internal rattle. You can here it during start up and lightly on idle

 

If the rattle is inside the cat, it's probably the filament. That's a sign of a used up cat. Would need replacement if that's the problem.

 

Yes: Cat converter rattle
One other thing comes to mind with rattle from underneath: check the clearance of your transfercase to skidplate. If too close/making contact, check your engine and trans mounts. Can cause rattling (usually worse in reverse). Just another cause of rattle in that area.