Chevy Avalanche Sub Box

In my avalanche, I have two 12's in a ported box in the truck bed and its sounds great in the front seats. I removed the midgate. But in the back seats the bass is bad sounding. You can hear port noise and rattle. I noticed that if i moved the box closer to the tailgate the bass get a little better but not the best. My port is facing the tailgate

It's because when you're in the rear you are close enough to the sub enclosure to hear its deficiencies.  But you know that.  When port noise is apparent, it means that the port diameter is too small for the system.  The rattles are most likely areas of the car's body in the close vicinity of the enclosure  As long as it sounds good in the front you should consider leaving it where it's at and fix the problems.  One is the need for a better enclosure with a larger port, which means the overall enclosure must be built larger.  Second is to apply sound damping on the sheet metal in the areas where you hear rattles.

I had the same box in my previous car and never noticed the port noise from the back seats and when I put it in the new truck I shocked at how bad it sounded from the rear. I can scrap the box, I built it based on my cars dimentions and guess thats where I'm messing up at. I built the box to the volume of audiobahns specs for their everyday basser class and your saying build it bigger with a bigger port.