Noise and vibration when cold

I'm getting this info second hand but my sister's 92 XJ has a bug. When starting and only while COLD, moving or not, bad vibration/noise/rattle from "under the vehicle." Worse when accelerating, deaccelrating, but once warm... problem "fades away."

I'm thinking it MUST be in exhaust somewhere?? Has an AW4 and a 231. Couldn't be a torque converter could it??

Any ideas??

No one...