Help from some genius!

Well I have a 1997 Ford f-250 4x4.

I am driving down the street and actually thought that some kid was trying to pass me on my left on one of those gas powered skate boards. It was very loud and startled me. As i let off the gas the Jake break type of sound slowed with the speed of my engine. OH OH!!! its my truck. I quickly turned and pulled over and as i was getting ready to stop there was deffinate thump, and then the noise stopped??? I checked tires, belts etc. Thought that maybe something had come off the truck, Broke a belt?? something but all was in order. Drove it a while and the noise was gone. Been going to and from work for a week and suddenly it hits again. This time with two thump, thumps. I was going streight both times and not very fast, maybe 35 mph. Does any one have an idea what this is. I cant get it to happen at the mechanics and they look at me like i am nuts. Hard to describe the sound. some where between a gas powered scooter, a jake break being used and a gas model air plane but ONE HECK of a lot louder!!! Any ideas…???

I would look for a weak link in the exhaust path.

A branch or piece of trash stuck in a wheel well or near the driveshaft? Maybe a metal or plastic shield occasionally contacting the ABS ring?