Rattle when firing up and shutting off

I have a 98 xj sport. Every time a fire up my motor ther is a rattling noise coming from undernieth the vehicle. I just had a new tranny proffesionaly installed 300 miles ago. Where do I start?

Check your cat. Starts to break down on the inside and rattle. Hit it with your hand when it is cold and see if it rattles. There might be a recall on this, not sure.

That or check your T/C shifter linkage.

check the exhaust, it's probably touching the crossmember.
the tech probably let the exhaust hang loose when the transmission was out, and its bent lower now.
I had this same thing happen after an engine swap, a few pumps with a floor jack under the converter's flange, and no more rattle.

The wifes 96 did that when the Cat was falling apart internally and then again when the inside of the muffler was rusting out (they often rust from the inside out). In my 88 the exhaust end was a little too near the gas tank and would make contact at start up.
The crossover (down pipe) could be too near the tranny/transfer cross member. You almost have to raise the exhaust a little higher than you want the finished height to be for clearance and then tighten the clamps, it settles and/or relaxes some over time. A loose, worn or broken tranny mount can cause the cross over pipe to contact the cross member also, on early models (I'm not sure about the 98 though). If the cross over pipe is hitting the cross member, it often happens driving over bumps also.

Also check the parking brake cable - might be resting on the muffler. Mine was doing the exact same the other day, and a quick check confirmed this as the culprit.

i broke my tailpipe hanger off a while ago and my muffler rattles on my floorboard sometimes. crawl under and give it a look.

OK....I get this at idle. It's kinda hard to hear, but I can feel an intermittent vibration coming through the floorpan. Exhaust against tranny crossmember, you say??