here are some thoughts, I would double over all nuts and bolts in the general area, could possibly be trans mount cross member bolts.
I would definitely run it with the axle tubes on jackstands and wheel chocks in the front and back of the front tires, WITH THE CAR POINTING OUT OF THE GARAGE with nothing in front of it, to see if noise is there with no road vibration.
When the noise is present You might want to step on the clutch and turn the ignition off to see if noise is still there with engine off.
If you decide to take the speedo gear out you could probably jack the back of the car as high as you can get it to minimize loss of oil.
While in there you should be able to see / poke at the vent baffle tin that is in there to see if its loose.
We've done the jackstands under the rear axle bit already; I "drove" the car that way while a friend bravely got down and watched from the side, but the car didn't make the noise on those stands.
He reported that the driveshaft looked remarkably smooth as I wound up buzzing the car all the way up to 70mph.
Made me nervous, I can tell you, even though we used 4 jackstands under the axle.
It only occurred to me later afterwards that the noise only starts after the car has been driven over 5 miles or so, so I reckon things have to warm up somewhat before the racket appears.
I've also already done the speedo pinion removal rhing; it looked perfect when I took it out (and yes, considerable fluid is drained when you do this that has to be returned). Further, the speedometer itself is one of the smoothest operating ones I've ever seen in an old car; GPS confirms I've got the right speedo pinion, too.
I tried speedo pinion in, out, cable on, cable off.
Nope.
When weather permits, I'm going to get a local shops' owner to go for a ride with me so he can hear it. He specializes in Mopars and has restored several.
If he can't come up with anything, I reckon I'll be left with little recourse but to swap out the transmission. :-(
