Driveline vibration

[72skytop]

I have 71 Cheyenne big block 402 with a 400 trans. It has 182,000 miles on it and I have driven it with this vibration since I got it in 1991 with 130,000 miles. I keep thinking as I rebuild things, I will find the problem, but I don't. In fact, here lately it seems to be getting worse.

Here is what it does:

It is kind of a droning that comes in and out when it hits a certain speed and quits when you get it going fast enough. It starts to come in at about 40 mph and quits at about 52mph. Never feel it again after 60 mph at all. The main point I want to make in describing this is the fact that once it starts doing this at 40, it is like a droning that vibrates anything that might rattle about every two seconds. Then after another couple of seconds, it vibrates for another two seconds and continues this off and on vibration until you get it over 60 or slow down below 40mph.

I have replaced literally what I think is about everything on this rig. I have had the engine out twice for different reasons and had it balanced both times including the flywheel of course. The harmonic balancer was replaced with a new GM part. all the pulleys where sandblasted and repainted. When the trans was rebuilt, the torque convertor was replaced. It had a 3.08 posi rear in it and I found another rearend with a 4.10 posi and I switched the entire rear axles. Still makes the vibration. One thing did change when I changed the entire rear axle with one of a different ratio: The same sound and vibration, but came in at a different speed. With the 3.08 axle the drone came in at about 45 and left at 55mph. That should tell me something, but I don't know what. It has the original two piece drive shaft which has had all the u joints changed and I have had both shafts balanced TWICE. I also replaced the carrier bearing. I am wondering about the alignment of those shafts. I have just aligned them by eyeball and what I could do with a what appeared to be a straight yardstick. I have even removed and rotated each driveshaft 180 degees one at a time. I have service and overhaul manuals for it, but I can't find anything in there about a an alignment procedure.

Any ideas or suggestions would be greatly appreciated. I never thought I would see the day when I would not want to drive this truck, but it really not a pleasure to drive anymore, despite its good looks.

Thank you.