I have a 90 Formula with the 700R4. I have noticed that during cruising at 60 mph with little to no load(downhill) that the tranny feels like it is rumbling. When throttle is applied or under a load (uphill) it goes away. The tranny shifts fine and shows no other symptoms. I also have a Spohn torque arm with the spherical rod end at the crossmember. Could it be coming from the rear and being transmitted through the crossmember? This problem is not a constant thing either. Somedays it occurs and some days it doesn't. Anyone have any ideas?

 

That is usually caused by wear in some bearing having to do with the drive shaft. Either a U-joint, or the pinion bearings (could be the nut is loose), or the extension housing bushing in the trans.

 

Any way to tell which end it's coming from? I figure the pinion will have to come out to replace the bearing there but how difficult is the extension housing bushing to replace?