It sounds like you are getting air into the exhaust system, But first make sure it has a full tune up. To check for exhaust leaks with the engine idling start at the head to exhaust manifold joint look and listen for leaks and work your way back to the end of exhaust pipe. Pay close attention to the connections before and after the muffler, Make sure that you have a tail pipe to the rear bumper or tailgate, if nothing else it moves the pops and farts to the back of the truck.
