knocking noise from rear

I have a knocking noise coming from the rear of my 2010 titan SE 4wd. It is only when the truck is moving and seems to follow the wheel revolution. Faster you go faster it knocks. Once you get some speed up it almost seems to go away but I think I just cant hear it over the truck and road noise. I have aftermarket XD revolver 20's. My right rear tire has a slow leak, took it to a tire shop and they said it had dirt in the bead. Still leaks air but not as fast now. What I have noticed is the higher the tire pressure the louder the noise. I hear it the most when towing. If I air the tire up to about 35 psi and tow the boat the noise is very loud. When making a left turn its louder than a right turn. When I make a right turn it seems almost goes away sometimes. Thanks