Reedcage

The rattle often comes from the speaker being pushed to the point where the speaker over excurses or moves beyond it's normal range - and causes damage. It will still work but no longer has the original "damping" and cone control. It's difficult to diagnose a speaker that seems to be working normally and the damage usually occurs over time. Use a sine wave tone generator (you can download all kinds of these for free to use on your smartphone) plugged into an input channel on your mixer that's connected to your PA - start at a fairly low level and SLOWLY and gradually"sweep" the frequency from 40 Hz to about 180Hz in 10 cycle increments at a time and listen carefully for any rattles, scraping sounds, distortion, or irregularities coming from your subs at each frequency. Problems will usually be emphasized or more apparent using this test method vs trying to diagnose issues during soundcheck or performance. It may need to have one (or more) speakers replaced with new units OR getting your tired speakers reconed by an experienced technician that is factory trained and authorized to do it. (Way less expensive)