I've read a few threads about similar situations but I don't see a lot that matches what I have going on. I have an 06 Titan with 46k miles on it. Recently I noticed a rattling or knocking sound that starts at 55 mph and continues up to 60 mph. It only happens at the bottom of overdrives rpm range, right before the trannie is ready to upshift. Generally it happens when I have decreased speed from 65 and need to slowly start speeding back up. It does not happen if I rapidly downshift or accelerate steadily through 60mph. Does not seem to be coming from the engine compartment. When I took it in for service today they pulled some rocks from in a skid plate and tightened a few things down with no success. Any help or ideas are greatly appreciated.