Ok here is the problem, I just joined. I had the same problem, it sounds like one of those old metal party clickers when you go over bumps that cause flexing of the chassis of the truck. I took it into the dealership 3 times and they got the brilliant idea of replacing all of the cab mounts, the grommets that are used in the attachment areas of mounting the truck body to the chassis. A full day and when I drove it off the lot - a big frigin' nothing! Same noise and problem. I went home and after spending a few hours underneath I found it to be the metal box molded into the body of the cab where it connects through the gromet to the chassis. It is a weak, flat metal box that flexes and acts like that metal party clicker I mentioned above. How do you make a flat metal surface stronger? You add ridges or grooves to it. I used a large bronze punch and hammered grooves into the sides and bottom of the "box" to give it better ridgidity, then usede a rubberized undercoating to reseal the area. I also used a spray foam to fill in any of the hollow boxes built into the botttom of the body of the cab area. Not a 100% fix because you actuall need to weld a better, thicker metal in that "box" area but it is 95% better. I only get that click / rattle once in a while. I hope this helps you as I too was going nutz over this problem. Afterall, isn't that why we purchased toyota and not any other pick up?