Totem Storm Subwoofer

I've had a Totem Storm for about a year now, only recently have I plugged it into the SW out on my Marantz AV8200 Receiver, previously I was using it to run my front monitors in "large" speaker mode directly from the amplifier.

I have noticed more and more than it "rattles" on the inside, hard to explain the sound, but is this the sound sub makes when it can't meet the frequencies coming from a song or movie? Ie: watching a sci fi movie with a very low hum engine noise as a spaceship flies by merely results in a tinny rattling sound, and not a full, bass sound.

Any way to know if I'm just reaching the lower limit of the subs ability, or if it is damaged?