My 2009 LT has a rattle in the back of the car only when in gear. I got under the car and checked for anything loose but found nothing. The noise seems to come from the roof. I made sure there was nothing loose in the storage well and that the rear seats were firmly engaged with the brackets when fully open.

Could there be anything in the ceiling/roof that might come loose and make a rattling sound?

 

Is this with only the rear windows open? Everything in the spare tire well secured? Rear bumper cover loose? Does "when igear" mean moving only or sitting still in gear?
Do you have a roof rack? Is it loose? It is screwed down into rivnits one may have popped out.
Is there a rear antenna? Maybe something came loose.
Is the cargo light firmly attached? It is attached to the roof with a magnet, it holds the hjeadliner up.

 

Hatch interior plastic panel.
3rd brake light panel.
Hatch itself. (Strut, weatherstrip missing, misaligned/defective latch assembly.

Or any of those things that Donbrew mentioned.

 

upholstery of the gate - you need to move it with your hands, you may feel movement;
rear stop-signal bulb - if it bursts, the glass dangles freely inside the lantern;
rear window broom - it may not be pressed tightly;
the gate itself may not be pressed - adjustment;
inside the gate (between the metal and the upholstery) there are objects - a washing system, wiring, etc. Some of this could weaken or fall off;
the area where the gate adjoins the opening also has upholstery - it also needs to be examined;
you can’t describe everything, just meticulously touch everything with your hands, listen while moving.

 

The noise is there with the rear windows closed. I checked the spare tire well and nothing is loose. The noise is heard only with the transmission in gear with the car stopped; there is no noise when the car is moving. The antenna is in the front, just above the windshield. I checked the lights in the headliner and they are not loose. No roof rack. I will check the rear bumper cover.

 

I will check the other possible causes mentioned.

 

Look in the front passenger wheel well. The mounting bolt for the Serpentine Belt tensioner is visible. If it is sitting low in the access hole, your front engine mount is worn out, and vibrations may be transmitted to the unibody. With the transmission in gear, the idle speed drops slightly, and the vibrations from the engine are more pronounced.

 

Had the same issue with our 2008 LT, would rattle from rear of car when idling in or out of gear (automatic). Found license plate bracket vibrating with plate and just touching it made the noise go away. I tightened up the mounting screws and jammed a piece of sticky Velcro behind the plate. Fixed!

 

When it came out of the factory there were 2 rubber dots under the plate. They disappear quickly.
I did the same thing.

 

I’ve noticed the rear plastic panel over the battery and spare tire area loose . A few well placed pieces of closed foam weather stripping tape cures that.