I have a 2007 RX350 with a rattle in the rear of the car that has stumped 3 mechanics. Every time I ask a mechanic to find out what it is, they have me replace something. I've replaced the exhaust mounts, rear strut assembly, and have removed the spare tire shroud. Nothing worked. It's getting costly paying for repairs that aren't needed trying to figure this noise out.
It is a rattle or wobble that is very noticeable when driving slow over any small bumps, dips, or uneven pavement.
Ive checked the compartment in the rear where the jack is located and made sure nothing was loose or rolling around. I am out of ideas. The car runs and drives great except for that darn rattling noise.
Has anyone had this happen? What could be some things to look for? I'm stumped.

Did you remove the entire compartment tray the jack sits in? There's a few things underneath that might be bouncing around making noises. There's a CD-changer for the DVD/Navigation (if applicable) and I think there's a stereo amplifier under there, too.

Somewhere in these forums there's a thread with pics detailing the procedure to connect a hitch/trailer wiring harness (sorry, I can't seem to find the one I'm thinking about). It shows how to remove those plastic trays to gain access underneath. Might be worth the time to check under there.

Have them check the rear knuckle bushings.
Raise it up and check it with a pry bar if there is any play.

check for loose heat shielding on the exhaust

I can't find the link either. Can anyone help please.

Check the rear hatch plastic

does the noise sound like a plastic water bottle hitting your door?

Not really. It's more of a wobble sound like something in the rear area is loose or something. I wouldn't describe it as a plastic sound.