I have had some success on the pre 1969 chassis in working with washers or spacers to adjust the angle of the transmission to the driveshaft, IE: lowering the rear mount incrementally to put the transmission and driveshaft in the angle that they "like".
That is helpful, and actually I think we've discussed this before. You confirmed for me that the more modern transmission-rear-bushing (also known as the 'hydraulic bushing') which is looser and has the inner sleeve offset downwards, causes the rear of the transmission to sit between 1/4" and 1/2" higher in the car, affecting the working angles of the donut and U-joints etc.
So I tried spacing the trans-mount downwards both 1/4" and 1/2" and neither made any difference I'm afraid - that was very disappointing because it was the best theory I had heard in a long time. I also liked the idea because the vibration does seem affected by the 'ride-height' of the rear of the transmission - I bought a set of those yellow, polyflex inserts that you can shove into the voids in the hydraulic bushing (thereby slightly lowering the trans) and the main problem..very low frequency vibration..did seem reduced - but newer and even more annoying buzzy vibrations from the engine/trans were introduced so I gave up - might be time to re-visit this idea again.
To the other question asked...This is road-speed related. The car is up on jacks with the rear axle on the stands to simulate correct diff-driveshaft angle. Very low frequency, very high amplitude vibration experienced at about 60mph+. It's hard to talk in the car due to 'thrumming' vibration - it's a bit like when you're on the freeway and the sunroof is open the wrong amount.
Some other experiments I've done - may trigger some ideas...
- Remove the rear wheels...still vibrates (i.e it's not rear wheel balance)
- Remove the rear axle half-shafts...still vibrates (nor is it a bad rear halfshaft)
- Remove the donut (ie disconnect the driveshaft)...no vibration (suggests the problem is between donut and diff, but the transmission is completely unloaded here, so the experiment is suspect)
- split the driveshaft at the center bearing (ie disconnect the rear half of the shaft...no vibration, but again, front half + trans is no longer loaded)
- Hoseclamps on the rear half...still vibrates, possible to slightly improve, and drastically worsen vibration, suggesting the problem lies here, but could NOT tune out the vibration altogether (this shaft is perfectly balanced off the car...so I don't like hanging hoseclamps on it, but I'm desperate)
- Put foot on clutch while vibration is happening - it does not go away.
I spoke to Dan Marvin yesterday, who told me a story of how he once bolted a driveshaft together wrongly and damaged a flange, and the shaft was never the same - and he ended up replacing it. Very interesting...the flanges (a little like grandpa's axe) are the only original thing I have left on the shaft, having replaced tubes, UJs, bushings and bearings. Maybe I just need a different axe.
again - any suggestions gratefully received. Thanks,