Sub Question?

So i got my FI Q back from a recone like 2 weeks ago. and since it's been hooked up i hear like a rattling type of sound when the bass hits during the majority of songs (some sound fine usually the loud bass songs) i went next to the sub to hear where it was coming from and it sounds like it's coming from the sub itself any idea as why this would be happening? Also if i put my back seats up (my box sits flush with the back seats so the sub faces the cabin while i drive so with the seats up the sub is touching the back seats limiting it's excursion) the sound dissapears and it sounds like clean bass on any song but since it is limiting it's excursion it is noticeably quieter and loud bass songs i can't play (well i could but i feel like i'd be damaging the sub)

so anyone know why the sound is their?

and 2nd is it harmful to the sub for it to be restricted with it's movement?