My xj recently hit 210k miles. There has always been a rattle possibly in the transmission while going at certain speeds. Now there is a much louder rattle while going in reverse, turning, and sometimes when in idle. My brother says it might be the transmission and could possibly not last this summer in the texas heat so im wondering if repairing the transmission is an option or try to buy a new car.

 

Figure out where the rattle is coming from first. There are all kinds of things that can rattle on an old XJ. As long as the transmission fluid is changed regularly, the AW4 lasts a long time. It could be the transmission mount. If the transmission mount needs replaced, it can allow the exhaust to hit the support cross member. You should get underneath your XJ and grab parts and see if anything moves. Then, if something moves, can it hit something. Worth a check of your motor mounts too. Once the motor mounts go, the entire motor/transmission can move and cause things to vibrate against each other.

 

LH engine mount rubber gets completely flogged out, check with a mirror, trans mount sinks down too

if something is actually "banging" inside yours trans, it wont last long at all, make it difficult to sell

a very close visual inspection underneath will usually reveal spots that are close, there will be a witness mark if they bang together

 

Also, check the shield for the catalytic convertor. If just one tack weld fails, it will rattle, usually at lower rpm, like shifting from park to reverse or vice versa. Its a resonance thing.