Now that I have new mufflers on the truck I can hear all the creeks and groans the old girl makes. When driving down the highway at 55 (or whatever speed) I can hear a sound that sounds like a transmission in a manual truck if that makes sense. Kind of a whining noise. When I let off the gas it stops. I think it might be the rear end bearings cuz it doesn't get worse if I jerk the steering wheel left and right putting more pressure on the left and right bearings. I probably should have checked the bearings when changed out the rear end seals.