Please help me diagnose Brake/Driveline problem

[Bullet4x4]

Hi,

I have an 88' K5 Blazer with the 700R4 and automatic hubs. For quite some time, I have noticed that when I stop, and then accelerate, I will feel a bit of a budging/pop/clunking noise when I accelerate. It just happens once per stop cycle. Sometimes I will stop at a traffic light, and while sitting still, I will feel the whole truck move slightly as I hear the clunking noise. Most of the time, it occurs just as I begin to accelerate. It seems most pronounced when I brake hard.

Is this a problem with my drum brakes or something in the drivetrain? The noise seems to come from the rear of the vehicle, although I haven't been able to pinpoint it from the outside. I first noticed this problem 6 months ago, and it was very slight. It is much more noticeable now, but it has not become worse over the last few months.

I intend to have the truck looked at soon, but I would like to trouble shoot this as much as I can before hand so I know what to expect. Any suggestions would be greatly appreciated.

 

[ryoken]

sounds like a shoe hanging up...

 

[OffRoadTek]

I have a 89 Jimmy that does the exact thing. I just got it from my dad because he didn't want to take the time and money to get it to pass the emission inspection where he lives. He said he couldn't find the source of the clunk and I haven't really looked for it yet. He did go through the rear suspension bushings but I don't think he ever looked at the brakes. Post if you figure out what it is. I won't be able to look into mine till next week.

 

[Bullet4x4]

I finally got around to taking my truck into the shop after the symptoms became progressively worse. I still had a clunking sound from starting and stopping, but I was also noticing alot more vibration when the truck was at cruise speeds. My local repair shop said that my U-Joints were on their way out and replaced them in addition to repacking my front wheel bearings (seperate issue). I was relieved to find that I no longer had the clunking noise, and all the vibration issues stopped as well. I hope this helps you guys with other mystery noised coming from your drivetrain.

 

[Fordum]

Folks, I had that problem for a while until I finally found it. NOTE: This will not be applicable for a lot of you.
My old truck had a slip joint in the rear driveshaft. Since I had fairly stiff
rear springs, I did not get a lot of suspension motion when driving down the highway. Over a period of time, the splines in the slip joint wore just enough to let the splines twist and not be lined up. When I would come to a stop, the rear axle would try to move forward slightly on the springs, but the spline being twisted by the automatic tranny torque would not slip.
As soon as I took my foot off the brakes, the twist would relax, and the splines would line up letting them slip with a bang.
Sometimes, it would feel like I had been hit by a car.

Greasing the joint would help, but it would come back.
I finally had to replace the joint. Since I am blessed with a really good local machine shop that has everything necessary to build driveshafts, including a balancer, it was not too bad.
Mine had the slip joint on the back end from the factory. I had him build me one with the slip joint on top so it would stay out of the mud.
Made a big difference.

This problem is not as common on trucks with slip joints in the transmission or transfer cases, since they stay oiled and out of the dirt.
But I have seen one on a high millage truck with the protected joint.

The easiest test, is to grease the slip joint and see it that makes a difference.

J.