99 chevy sub install

I have an extended cab 99 chevy truck. I installed (2) 12" DB Drive subs. They are in individual truck boxes slid under the back seat. The subs probably have about .5 cu. ft. apiece. The problem is they make noise. I've tried mounting them different ways but I can still hear the noise coming off of the sub. Maybe they dont have enough air space or I could have defected subs. They are powered by a 2000 watt (probably alot less) Digital Audio amp.

Describe the noise.

Its an aweful rattling coming off the front of the cone. Its kinda hard to explain I just know that it sounds bad. I have installed the same line of subs in several different cars and they all sounded great. I just want to know why mine sounds like it's bottoming out and rattling. Would it make a big difference if I bought the custom made box for the truck and scrapped the individual truck boxes.

Not really familiar with those subs but 0.5 cu. ft. seems REALLY small for a 12" sub to me. I have (2) 10" Alpine type R's under the seat fo my 2001 Silverado Xcab (each has 0.75 cu ft.) (got the box from www.supercrewsound.com) and they sounded a little noisy until I lifted the box off the carpet a little farther (it's a downfiring box). The place I got the box from makes them for 12" subs also (same size enclosure) with no need to raise the seat if you're looking at going that route.