Sub Rattle

I currently have the PSW 550, I only bought it a month or so ago. When listening closely to the sub, with the main speakers turned down, I've noticed a faint rattle. I previously had the 350, and noticed the same thing. Has anyone else ever noticed this? Is there anything to remedy it? I know it's nothing in my system, because I noticed the same problem hooking it up to my friednd's sytem. Also, the subs not maxing out, it's barely moving. The volume on the back is around half. Listening to it normally, I can't notice it, and the sub sounds great. It's only when I listen closely to the sub.

I'm thinking about shelling out some more cash and buying a Velodyne SPL. Anyone had any experiece with these?

Thanks
GuitarheadCA