Have a 98 Blazer with 4WD that has an intermittent humming / clunking.
The noise (almost) always starts at highway speed after driving for at least half an hour. It starts as a quiet consistent hum, then as the vehicle slows the noise gets louder and lower, when I near stopping it turns into a clunking. It seems to be coming from the center of the vehicle. When I stop the noise stops. Most of the time when it starts moving again, the noise is gone. The noise definetly changes based on the speed of the vehicle and does not matter what forward gear it is in. Also makes the noise in neutral.

Occasionally, it starts again but it always stops eventually (so far). The noise has occurred maybe six times in the last few weeks. I did notice that one of the cv axle boots is torn and leaking grease. Does this sound like a possibility or is this something worse like a torque convertor?

Thanks for any thoughts.

BB

 

It is a possibility that the CV axles are starting to go. But if it's from the center of the vehicle I would also consider the drive shafts and their u-joints, especially if there is vibration. I wouldn't really think the torque converter could cause those problems, especially if you haven't noticed any difference in starting from a stop and maintaining speed. You say it happens in neutral, too. But I am guessing this is while you are still moving which means the axles are still turning. As long as it's not doing it while you are in neutral and staying still, you can kinda rule out transmission problems.

 

Yeah the neutral & cv joints/boots = is it moving? If not then it's not the joints but you should replace them soon.
Does it get worse in 4wd? Is the trans topped up & has been serviced lately? IE: fluid dropped, filter replaced etc?

 

I'll jack it up and crawl under to check the cv and the u-joint and driveshaft. However there is no vibration at any speed associated - no loss of power either.

When the vehicle is stopped, the noise is gone.

Tranny was drained, filter replace less than 5K ago.

 

I have the same issue, except I put in a new tranny and the noise is still there. put grease in the rearend (which was almost empty) and the noise is less but still there.

 

front axle partially engaging? would cause the clunk when you come to a stop, try disconnecting the vacuum hose to the actuator and plugging it. see if the noise goes away.

 

I have the same model and year blazer and have the exact same problem. My mechanic can't figure it out and says everything under the vehicle is fine. Has anyone figured out what this problem is?

 

As mentioned, disconnect the vacuum line at the front axle actuator, (under the battery tray) plug the vacuum line and drive it. If the noise is gone, the transfer case vacuum switch needs replacement.