So, I've got the same problem this guy did - http://www.polkaudio.com/forums/show...ng+cabinet+sda
I don't like digging up old threads, so I figured I'd start my own.
If I grab the back of the cabinet slightly below mid-level, the rattling stops. If I just hold my hand there, I can feel air being forced out of the cabinet. Note that this is only truely noticible at high volumes, but I'm sure it's effecting the sound at lower volumes too.
Since I'm about to tear into them to install my rings, I figured now would be a good time to fix this. Suggestions? Just a bead of glue on the inside?