The Stovebolt Site Forums The Shop Area Engine Shop Odd vibration

Greetings,
I must compliment you all on having the best BBS that I have ever found. Good constructive advice and plenty of it.
I have a question and I tried not to make the newbie mistake of not looking in past posts but I couldn't find anything like this.
My truck is a 51' GMC 3/4 ton. with what I think is called a platform bed? It has the SM420 4 speed and a 1960 235. It also has a two piece drive shaft with the front being the tourqe tube variety.
At what I'm assuming is about 35 or 40mph it starts to vibrate and the faster you go the worse it is... the strange thing is that if you let off the gas quick and then ease back on sometimes it will dissappear and you can accually hear the person next to you talk. I'm assuming this is a drive line problem but I wanted to run it past you all before I started to tear stuff up. Maybe my tearing won't take so long that way? :0)
Thank you

Welcome to the Stovebolt Grampes

I am suspecting the center support bearing may have a worn rubber in it,,,,,(That sounds bad)
I mean to say is the rubber surrounding the center support bearing on the driveline is worn or loose.
Be sure to have the wheels blocked before crawling under the truck.

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You may also block the front wheels, rais the rear wheels off the ground and spin the driveline and wheels by hand. Listen for noises and where they come from.

Beside the driveshafts, you might check your cooling fan. I struggled with the same situation even after balencing the shafts and bearing work and finally found a cracked fan blade that was flexing at a certain speed and transmitting the vibes through the engine/trans. I replaced the blade before it broke and the vibes went away.

I wish I could check both of those but I'm in colorado and my truck is in Oklahoma..... frustrating... very frustrationg! In two weeks though I am going back and might be able to bring it up here. 800 miles in a unrestored 51'? We will just have to see. Thank you gentlemen.

After the hangar bearing I would check the splined section of the rear driveshaft for wear or damage. Saw a big 4x4 worn there once that vibrated so bad that you couldn't read the instrument panel. But I think that wear in the splined area would be a long shot.

I'm having problems finding a center bearing for the 3/4 ton. Chevy duty didn't seem to have it and neather did a couple of other places.... suggestions?

Thats is a odd problem. When you let off quickly what ever is worn is getting realigned and by easing back on the throttle it stays that way. I would check the u-joints and the center support bearing. Engine or transmission problems most likely would not come and go from a on and off throttle move, they would be more of a rpm problem. Joe

For the center support bearing also known as a carrier bearing, look at a local auto parts store talk to a knowledgeable person there some you ,,most younger guys have no clue,,, some do

My 2 ton does something similar. I have always assumed it is bearing trouble in the tranny. My noise is not there during acceleration or decelleration but there during coasting.

So I assume some shaft needs to have a load on it either way to not make noise.