I noticed a mysterious noise on my way to work this morning coming from the rear of my truck. I didn't notice it until I had the radio turned off and was sitting in traffic. I initially thought it may be related to the new wheels I just put on last night, but after listening to it more, it sounds like it may be related to the driveshaft or rear diff (truck is 2WD). I'll try to explain it the best I can. Since I initially thought it was one of the wheels, it sounded to me like something was loose inside the wheel. Like the person that mounted the tire unknowingly dropped something inside. It sounds like a rubber ball bouncing on a wooden table or a gentle knocking sound like someone tapping there knuckle against a wooden desk. The noise starts as soon as the truck begins to move and it doesn't have a consistent frequency. It has a random pattern to it. I can hear it at higher speeds too and it is most noticeable when I am barely pressing the gas pedal. It seems to go away when pressing the gas pedal further but that may be because my exhaust gets louder. It also does it at certain speeds when letting off the gas pedal and coasting, but then goes away as I slow down. I checked underneath the truck and I didn't see anything noticeably wrong. The bolts connecting the driveshaft to the diff look tight. I'm dropping the truck off at the dealer tomorrow to have the timing belt changed so I suppose I could have them look into it, but I don't want my bill to skyrocket (it's already going to be $1,150). Any ideas? FYI, I drained and refilled my differential last weekend but I can't imagine why that would have anything to do with it. Unless something got knocked loose or fell inside the differential when I was changing the fluid. I used a hand pump and stuck the hose inside to fill it with fluid, but that shouldn't have hurt anything. Crap...I wonder if the magnet on the drain plug came loose and is rattling around inside. That is kind of what it sounds like. Does this sound like a possibility?