We have an old Blazer with what I'm calling the 4 button (2WD, 4HI, 4LO, Auto 4WD) setup. Up until today everything worked correctly. Being summer I wasn't going through gears, it was simply in 2WD. I was towing the boat from my parents house to my house when I started hearing an odd noise. The best I can describe is that it sounds rotational, but isn't directly proportional to the speed. It was semi random depending on the speed but shifting into neutral while moving seemed to make it disappear. With the truck in gear and coasting down a hill it was also silent, up to a certain speed (maybe 35mph) and then you could start to hear it but not nearly as loud. Also as if a strap was on the driveshaft spinning with it and randomly making a noise.

I got the boat home fine, up hills etc no issues. The transmission shifts in its gears fine and was smooth. None of the noise was accompanied by any odd vibrations and power was fine. Going up a hill with the boat did not increase the sound. When I got home I unhooked and took it for a spin. Same sounds. I tried the transfer case and selecting 4HI it flashed but never went solid. It did say it went into 4LO though, and driving forward it was in that mode, with no odd sounds. Typing this now I'm going to try it again and get going as fast as I can in 4LO and see if once engaged the sound is gone.

To me its also as if something inside the transfer case is touching or failing to keep it out of 4WD while it 2WD, like a gear sporadically touching as I'm going along. Thoughts welcome. This transfer case isn't horribly expensive but if a a failure of something else could cause this (even though it didn't happen while switching, just being in 2WD it started) I welcome it. I may try and get a video for sound.

This is a video of the noise.
https://www.youtube.com/watch?v=Kzt8...ature=youtu.be

I did try the 4WD modes again, neither will ever stop flashing, despite actually moving into gear. I hear the module in the truck and underneath clunking into place and it for sure (4LO for certain) is in gear I can tell when driving. The noise persists even when its engaged in 4WD. Only in reverse do I hear no noise.

Now we did have a driveline vibration fixed recently. The U-Joint on the driveshaft was done as it was vibrating something fierce. Now I don't know if during the month or so of very limited driving that vibration damaged something in the transfer case. I do know the driveshaft is fine.

 

Problems with the vacuum system can cause partial engagement of the 4 wheel drive. Did you get the "no floor heat" issue solved? That runs on vacuum as well. If not, they could be connected.

Next cheapest thing would be replace the fluid in the front diff and the T case. Carefully examine the old fluid for debris. I use a magnetic pick up tool to stir around and see what it picks up.

 

I didn't as the heat of the summer is up so it wasn't really a concern. I did check the vacuum on that system and it seemed good. Is there an easy way to "trick" the system? I do have a small vacuum pump I could pull a vacuum on it to see if it will fully engage again.

I can tell you this whole vacuum business makes me yearn for electronics and solenoids!

 

Rubber tubing is cheap. Trucks with rotten vacuum lines that are plugged with bolts never seem to run quite right. In my experience, by the time I figure out a system well enough to cheat it, fixing it correctly makes more sense.