Question about effects of having a driveshaft that is out of balance. 1967 GTO.
 

I have been noticing that after driving car for a while there is a chatter/clanking sound coming from the rear end. It only does it when the car is warm. When I come to a stop at a traffic light it is really noticeable. It is also noticeable driving at slow speeds (15 mph or less). My car is stored inside the basement when not being driven. When I want to drive it, I put it in neutral and push it out. I don't start the engine. I noticed when I do this there is a binding feel when it's being pushed out. I took the car to a local shop and they put it on a lift. I put the car in 2nd gear and the mechanic showed me the drive shaft was not spinning true. Both rear wheels also did not spin evenly. Could it be that my whole problem is the driveshaft is out of balance or bent and not a problem with the rear end. The car has a posi rear end. Any help would be greatly appreciated.

Driveshaft imbalance would not manifest itself at low speed, nor would it be temperature dependent (ie, you said it is worse when warm).

I'd say its in the rear end.

K

Keith,
Would having an out of balance driveshaft put strain on the rear end? If so, should the driveshaft problem be fixed before the rear end gets fixed? Thanks.

Unless it is WAY out of balance it will have little effect on the rear end. Unless your car has a limited slip differential both rear wheels will NOT spin evenly. Also a car that age that has a limited slip dif probably needs it rebuilt anyway. If you are the least bit handy pull the rear end cover and see if there is any indication of bad bearings or broken teeth. Especially check if any pieces of metal are in the fluid.

I pulled the diff. cover off. Fluid looked fine. Took a video of the rear end with the back of the car lifted up, spun the wheel and the gears seemed to move okay. I could replace the posi unit but I'm not sure that's the problem. There's a local speed shop near me, I'll just have them fix it.

I had a similar bizarre problem with my 66 Lemans with an HD 3spd and LSD. When I would roll it out of the garage it would occasionally give a big thud from the rear. It would also sometimes do it while turning at very low speeds such as in a parking lot. I suspected brake binding in the rear or a diff issue but couldn't find anything. Long story short, what it needed was GM limited slip diff additive. I had apparently forgotten to add it though I thought I had during a 4'wheel disc brake conversion. Drained the diff, added the additive and filled with new diff lube. Problem solved.

Without the limited slip additive, the limited slip friction clutches have too much bite and the thud is them being forced to release. At road speed everything is happening so fast and normal road vibration masks the symptoms so you don't hear it. I have been advised over the years to only use factory GM LSD additive and whatever diff lube you want.

As to the look of the driveshaft, the eye can play tricks on you and give the appearance of out of round. Ever seen a dirty tire/wheel look out of round on another car on the highway? As someone else said, it would get worse with speed or at least show up at the same speed consistently due to harmonics.

As far as the two wheels appearing to turn at different speeds on the lift, that is right, they will be different. Remember this is "limited slip" not a locker. Unloaded the amount of friction in the diff is small and the viscous coupling won't generate enough heat to equalize the rotational speed.

Try redoing the lube and put the GM LSD lube in first to make sure you have put in enough. I don't remember how much you need but that shouldn't be hard to find. It's an inexpensive thing to try before you spend a fortune. Just an idea.

Scott,
I used lucas oil diff. fluid when I put everything back together but did not add the additive. I've had the car about 11 1/2 years but this problem only started happening about 2 years ago. The only thing done to the rear end during the time I've had the car was the pinion seal started leaking real bad so I had that replaced about 7 years ago. I will put some additive in and see if that helps. Thanks.

Sounds like binding U-joints to me.

My drive shaft got out of balance, unbeknownst to me. Took it in to have transmission work done, shop said I needed a new front yoke. Got the car back and through 1st, 2nd and 3rd gears it was fine. Once in to 4th gear and above 45 mph there was a slight vibration between 45 and 50 mph. Got above that, it went away. By the time I realized it was getting worse, another shop looked at it because the one that worked on it previously had closed and moved. This shop said in the process of the old shop using a hammer instead of press to separate the u joint for yoke replacement, he hit and dented the drive shaft.
The vibration took out the tail shaft bushing, so $450 later for a new bushing and a new drive shaft, the vibration was gone.

Same exact thing with my '65 GTO about 30 years ago. After the driveshaft was fixed, I installed a new front yoke (old one was saddled) and new rear trans bushing and seal (Muncie 4 speed) and it's been fine ever since. It was just a small dent, too.

Reviving this thread. Still don't have this problem fixed. I put it on the back burner while I was trying to put the car back together. Now that the car is back together and drivable, I decided to take the driveshaft off and push the car out of the basement. It still has that binding problem. Seems to me the popping sound is coming from the rear of the car. Before I took the driveshaft off, I had both rear wheels off the ground and spun the tires by hand and as I was turning the wheel It would get harder to turn before it got easier again. I'm not sure if that is normal or not. I guess I need to add the limited slip diff fluid before I go any further just to see if that helps. Should have done that sooner before doing all this other stuff.

Sounds like a bad u joint. You should check it. Easy fix

That was going to be my suggestion. While I don't know alot about limited slip rear ends, I do know that they won't function properly without that fluid.

I gave some misinformation in my first post. I thought the problem only happened after driving the car around and it got warmed up. I then started noticing when pushing the car out of the basement after car was cooled down that there was a binding/popping noise coming from underneath the car. So, the problem is not temperature related. I'm taking it to a mechanic in a few days so hopefully he can figure out what the problem is. He is the one who looked at it a couple of years ago and showed me the driveshaft was not spinning true, and the left rear wheel was spinning out of balance. Maybe the left side axle shaft is bent.
Helmerrock,
The driveshaft was off the car along with the u-joints and when pushing it out of the basement it still has that binding feel/popping sound to it.

Jack up the rear end and wiggle each rear wheel. If you get ANY up and down or side to side movement, you have a failed axle bearing, along with a probable bad axle. BTDT with several first-gen GTO's. Your symptoms are classic.

Update:
Putting the limited slip diff. additive in the rear end fixed the problem. Pushing the car out of the basement is smooth as can be. No pop or binding feel at all. The fluid in the differential must have degraded over time. I don't know how old the fluid was. I drained the fluid about three years ago but did not put the additive in at that time, so the problem was still there. The driveshaft problem is not that noticeable so I'm leaving that alone for now.

There are 2 lessons here.
1. A diff lube that says it is for LSD doesn't mean it has the GM additive it needs. In the past, only GM dealers had the additive but now it is available from several sources.
2. As a now dearly departed old school mechanic told me when I didn't think his suggestion to add the additive was right either, "when in doubt, try the simplest, easiest and cheapest solution first. You might get lucky." RIP Jack.

Glad this all worked out for you.

Thanks Scott. Should have tried the fluid additive first.