Thanks to Cole C. for writing this helpful tip!
What's The Rattle?
Look through the shell carrier on the bottom of the receiver and see if you can spot a black metal disc. Did you find it?
That is the sound of the metal disc on your receiver recoil buffer after it has separated from its nylon shock absorber that is still stuck in the rear of your receiver (hopefully). You will need a new receiver recoil buffer!
Picture attached from a LAW-12, date code 1987, that has been fired all of four times in its life and is in mint condition. Do not trust the factory receiver recoil buffer to withstand the test of time.