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riverray1969 it would help if I knew the exact model number of that JBL sub you are referring to. With that info I can provide exact specs.
Basically, you need to build/modify an sub enclosure that is suited for the sub, the music you mainly play, and vehicle tuning. Generally, sealed enclosures will provide tighter bass response and get decently loud. Some would argue that it will give you greater sound quality. A ported enclosure will give you a louder more booming sound. That depends also on where the port is tuned (ex. 32 hz, 36 hz, etc ). You need to make sure that the enclosure meets the sub's airspace requirement as well. A customized well made enclosure is just as important as the sub you put in it.
For optimal performance there is really only one size for the box .Firstly, you get two different types of subwoofers, an open air, or closed air sub. If it is an open air sub ( hole with mesh in the center of the magnet) no ports are required, and the box should be sealed on the inside with silicone sealant. If it is a closed air sub ( no hole in the magnet) you need a 50mm port about 120mm long. Box size will be 11 inches x 11 inches x 11 inches. Hope this helps
I wouldn't suggest putting any subwoofers on your rear deck. put them in the trunk. and if you choose to put them in the trunk you can use either 10's or 12's your preference but for the airspace in that car I'd go for 10's in a to spec and properly tuned ported enclosure. Building them yourself always sounds better if you know what you're doing.
Polyfill is only used when the enclosure is smaller than recommended. Th polyfill makes the woofer think the internal airspace is more than actual. Your always best to build the box to the recommended woofer specifications of the manufacturer.
You did not specify what type of enclosure the subs are in or if they share the same airspace. But the sound you describe sounds suspiciously like the enclosure is at fault and not the sub itself.
Remove and reverse the subs in the box and test for the noise again. If the noise is still produced on the same side of the box, it's the enclosure. If the noise follows the sub, then you have a problem with that sub.
If the box is making the noise, and it's a ported box, you probably have "port noise". If it's a sealed enclosure, you could have either an air leak or flexing noise. In either case, repair the enclosure or put the subs in a new one. If the sub itself is making the noise, there probably isn't much you can do. Most likely some part of the voice coil assembly is touching where it should not.
Sealed box and somewhat large if they are 12's I usually try to get 3 cubic feet overall internal dimensions and generally the inside dimensions will be 13" tall 32" wide and 13"-14" inches deep. with no divider and make sure the box is sealed. Personally I like to phase the speakers backwards. Gives the bass a little more rumble.