04 blazer, every hour or so while driving it acts like something is trying to engage on the 4wd. loud, fast clicking noise heard from front end. When it does this I just push 4wd auto and it goes into 4wd and stops the noise. Minute later put it back into 2 wd and it will be fine for an hour or so. Everything else about 4 wd seems fine. Mechanic has not heard it and only want it when it acts up. I am BYard mech for years but new to 4 wd problems. Thanks for any help... searched many forums and no similar issue found.

 

are you getting any service 4wd lights on ?

 

No service lights or blinking control switch issues. Sounds almost like playing cards in the spokes of a bicycle.

 

Check vacuum solenoid near firewall. Sounds like you are getting constant vacuum to the actuator under the battery tray. Not advisable to drive too much in this condition.

 

I had the same problem with my 99. I'm not sure if this will help you or not as you have a solenoid on your firewall and I have a switch on my transfer case. I unhooked the vacuum line near the firewall on the drivers side and plugged the ends to keep it from happening on my way to the parts store to get a new part.

 

i am having somewhat the same problem with a 98 blazer in 2 hi it clunks like a mother from 30 k and higher but once i hit the button for 4hi it all goes away not sure what this is or where i shoiuld look to try and figure this problem out.

 

How do I know the vac solenoid is the problem? I found a broken vac line near the sol. going to replace it and try. actuator appears tight and sealed.
thanks,
Found a broken vac line at the firewall drivers side. Fixed it and has not acted up after driving for 1/2 hour. Still early but I am hopeful that might do it.
thanks,

 

https://blazerforum.com/forum/showthread.php?t=41226
That should answer your questions.

 

Got my fingers crossed for ya.

 

i am not to sure on what is going on with the truck so far i cant seem to find the problem i am going to keep looking