I have a 1994 Class C motorhome on a Ford e350 chassis with the 400 cuin engine. About two years ago I was having trouble with the speed swinging and the torque converter going in and out of overdrive. I could not run using the cruise control due to the speed swing. This occured above 60 mph and was accompanied by a loud roar apparently coming through the drive train. Between me and my local mechanic we decided to change the torque converter thinking there was a problem with the lockup mechanism. This solved the speed swinging problem and the problem with it going in and out of overdrive.
But, the engine / transmission roar is still present. It is limited to between 62 and 68 mph. The speedometer still swings around a bit, but I am able to run with the cruise control on. Once I get out of that speed range the sound nearly goes away and the speed levels out. At the same time I hear the roar, there is a buzzing sound somewhere in the area of the passenger side dash... I think. It is really hard to pin point.
In addition to the torque converter, we have checked the u-joints and changed the transmission fluid. We have not done anything with the differential. I wonder if it might be a problem with the speedometer cable/linkage or the harmonic balancer?
I am worried I have not resolved the root cause and will again have problems with the torque converter or some other drive train component unless I get the issue resolved. Any help or suggestions on where to look next would be greatly appreciated.