If you have any kind of subwoofer setup for the stereo in your Mazda 3 you likely suffer with annoying buzzes and rattles from your rear deck behind the rear seats.  These two videos demonstrate an easy and inexpensive solution to your problem:  Carpet under padding from Home Depot sandwiched between the plastic deck and steel structure.  While in this video he uses the inexpensive variety, I would recommend upgrading to the Eco-Foam that Home Depot carries as it is moisture resistant.

Part 1

Part 2