Weird rattle

so i'm just driving down the road in 4th at about 2000 rpm when all of a sudden out of no where there is this really weird rattle noise comes from the back of my car. It was pretty loud enough to hear over my exhaust. It sounded kinda like plastic rattling but i couldnt find any loose plastic or anything loose at all for that matter. Anyone else know what this might be?
It is definately from the rear of the car not engine.

ok so i guess i just had a sevier lapse of brain power cause when i went to look at it i realized that a socket that i had on a rail in my trunk while going over to a friends house to work on his car had fallen off the rail and had gotten between the spare and the metal in the trunk and caused the tattling noise. Thanks anyway for the help