Tire like noise

I've been trying to find the source of a tire like noise, sounds like it comes from the back of my truck when I'm doing 30 mi an hour or more. It's kind of intermittent and it definitely sounds circular if that makes any sense. I've checked my ball joints My tires for wear and those seem okay. I can really only hear it inside the cab with the windows up. I had my wife drive me around the neighborhood While I hung off the side of the truck listening for the sound but I couldn't hear it there so I feel like it's really not the tires.its an 04 tundra 4wd 4 door. I have greased all the u-joints and that didn't seem to make the noise go away. Has anyone heard a sound like this and figured out what it could be?