I get an intermittent noise from below the floorboards. It starts around 30 MPH and will go away sometimes if I back off the throttle. I'm thinking universal joint or driveshaft bushing. The engine is fresh, and the transmission has new bushings as of last spring, I would rule those out first. Never know though. The rear axle normally does not make any noise, or if it does, it is not loud enough to be heard over general T noise. I do know the rear axle has bronze bushings and not brass, it was rebuilt some years ago. The car seems to run fine otherwise, but it seems louder recently.
What would a bad u joint sound like? I would think I would notice play in it first. The drive shaft bushing? greasing it does not seem to help. The rear axle does have sufficient grease/oil in it,
It's no fun removing the rear end from a T, but that is the only way you can observe the u-joint. I've seen u-joints that are half worn out. Should you go that far, I would definitely remove the entire torque tube and examine every part in it. Most of the time the rear inner drive shaft sleeve will be cracked around the slot for the keyway. There are good used u-joints on ebay all the time. I would stay away from the new ones. The front driveshaft bushing was originally babbet, and there is nothing wrong with that, but it is possible it is badly worn too. Follow the Ford Service book for instructions.
"I get an intermittent noise from below the floorboards."
As Sportin' Life said, it ain't necessarily so. Rear axle noises often sound like they're under foot. When you have the rear end out to check the U-joint it's probably a good idea to take a look inside the rear axle too.
First rule out engine knock. With the throttle steady and the noise present, change spark lever position. If you retard the spark a bit and the noise goes away, your noise is most likely in the engine.
If that doesn't eliminate the noise, remove the inspection plate from the transmission and check for looseness of the planetary gears. Sometimes if the bushings are too tight when installed, they will stick on the pins and spin inside the gears which causes them to wear out quickly.
If that doesn't seem to be the cause, jack up one rear wheel with the parking brake forward in the high gear position and turn the wheel back and forth. There is usually some play in the differential, but if there seems to be some clicking in the area of the U joint, it could be a bad joint. You might be able to take out the 4 bolts around the ball cap and slide it back far enough to see through the grease hole with a flashlight while someone turns the wheel back and forth to see if there seems to be unusual looseness in the joint.
If these checks don't reveal the cause of the problem, you might need to take things apart one step at a time until you find the cause.
Unfortunately it could be one of many possible things causing the noise. They seem to be telegraphed to "under the floorboards"
Also check the fit of the square spline into the driven plate at the back of the transmission. Wear there could also cause a noise.
If the cap/ring is loose around the drive shaft ball causing play between the ring and the fourth main, that can cause some clunking but usually it usually occurs during starting, stopping or shifting from low to high.
It can be cured with a sheet metal shim.