HELP,98 Diesel Thumping sound towing on hills
 

I recently took a long trip with my 98 Ram Diesel towing a travel trailer for the first time and started getting a thumping sound coming from inside the engine compartment while pulling a hill in 5th gear and had to downshift to 4th and sometimes to 3rd to get rid of it. It seemed to get slightly better with the AC off but did not stop completely. It sounds like it 's located close to the cab, almost right in front of the dash, but I'm stumped as to what it could be. Any ideas?