I own a 2002 XL 2wd with a 4.2 and an auto trans. When I drive at speeds between 40 and 55 I hear a rattle coming from under the hood. When I hit the gas hard it goes away, and when I let off the gas it goes away. It almost sounds like pinging but it dosen't do this under load. I dont think it is an exhaust leak but I dont know for sure. I have looked under the truck and found no loose exhaust, or any sign of leakage. I have also tried to get it to make the noise when it is sitting in neutral but it wont. It only does this when I am moving. Has anyone else experienced this? Any input would be greatly appreciated.

Jersey Jeff