I was hauling travel trailers 50 miles to a RV show for a dealer today. While pulling the trailers down the interstate, I drove about 70 mph, but when coming back empty for another load, I was running 75-78 mph. A number of times while coming back empty, for no reason the motor would start making a sharp, rapid, knocking sound similar to the noise when you tromp on it at low speed. EGT's were normal, temp okay, trans okay, no lights, and there was no change in speed or rpm. Edge EZ was in #1 position all the time. The noise would eventually go away (5-10 seconds) if you held a steady speed, but would stop immediately if you let off the gas even just a little. Fuel filter changed 1500 miles ago.........What's going on???