I’m feeling a slight vibration at idle. Could it be a motor or transmission mount? Or something else?

 

The motor and transmission mounts harden and can crack over time. I have replaced mine 2 times. The first time was during my XJs early life when I would go to acceleration runs and load up at the line by pressing hard on the brakes and then applying gas to get off the line fast and this placed a lot of torque strain on the mounts. The second time was for age and feeling vibration through the steering wheel.