2004 Ford F-150 Vibration

I was towing my camper this weekend. She had to work quite hard due to strong head winds on the freeway. I traveled about 100 miles till I got off the freeway. Once off a short while I let off the gas while climbing a hill cause someone was trying to pass me. When I applied the gas there was like a vibrating/shimming noise. I let off the gas and reapplied it and she was fine. I thought maybe she was between gears. Then for the trip home everything was fine till I got into town and was slowing down for a red light. I didn't have to come to a complete stop but when I applied the gas she did the same thing at 2 seperate times. As soon as I hear the noise I let of the gas and then reapply it she is fine. There is no burning smell or anything? What is going on? Also when ever I tow I also have the OD off even on the freeway.

Michele