I've got this rumbling noise in the back of my 91 Camry. Sounds like it's on the left side. You can hear it at low speeds, going over little bumps in the road; sounds like something heavy is rolling around in the trunk, or bouncing around, but that's not it. The spare tire is screwed in tight.

The muffler shield was a bit loose, i think since the exhaust pipe broke off last summer and got replaced; fixed it on better, no effect. I yanked on all the rear axle and suspension stuff to see if something was loose, but hell, i don't know. Seemed solid to me...

 

Might check the sway bar bushings and end links, and upper strut mounts.

To check the upper struts mounts have someone drive the car over bumps as you listen to the upper outer area of the rear deck.