Hi everyone!
So, i have a 2001 4x4 blazer and as of late its been making a humming sound when i drive 60-80km and it quiets down a little when i reach about 100km. Any thoughts as to what it might be? It is obviously speed dependent.
Something else that i have discoverd tonight is when i put it into 4hi to test it out before snowfall it made a very loud clunking sound when i tried to turn left going forward and right going backwards. I then proceeded to put it into 4lo (done in neutral) and started driving very slow, it was obviously more torquey but also at one point it felt like it grabbed and then suddenly slipped (almost like a transmission slip) and then when i went to slow down it slipped just before i came to a dead stop.
I would really appreciate any suggestions or insight.
I love my truck to death and ive put in so much work into it this year id hate to see it go!!

 

I would start with a thorough inspection of the rear driveshaft u-joints. A u-joint that is just starting to go bad can cause a vibration at higher speeds.

As to the clunking noise, this may be caused by a problem in the front axle engagement system; that it is not fully engaging and that what you hear is the engagement collar engaging & disengaging as the teeth pass over each other. If left in this condition, it can eventually wear the teeth out requiring replacement of parts inside the front axle passenger side extension housing (see link below even though the description doesn't exactly match your condition):
4wd Front Axle Clicking When in 2HI - Goes Away when in 4HI/4LO

When driving around in 4LO, the transmission shifting can seem weird due to the lower gear ratio created by the transfer case in low range. How fast were you going when you felt this 'slip'? Did the slip occur at or near the same speed upon both acceleration & deceleration?

 

oops i didnt clarify where the vibration seemed to be coming from. its felt mostly and heard mostly on the front driver side, it has gotten alot worse now and the vibrations can be felt throughout the front of the truck.
the engaging and disengaging makes alot of sense because last winter it didnt really make a peep when i put it in 4x4 but when i started testing it out recently to make sure it was functioning its been clunking bad.
would this have anything to do with the "switch" or sensor that seems to come up alot? i checked my vacuum line by the firewall near the brake booster and its all plugged in so im assuming that part is ok.
do you think by doing a prealignment they would be able to tell me exactly what is wrong?
something else i noticed too was a few weeks ago, in 2hi i could spin my front driveshaft, 2 weeks ago, i couldnt, last night i could!
in 4 lo i was going MAYBE 5-10km and i know i know, dry pavement is a big nono :P same hop and slip speeding up and slowing down.
ugh im losing sleep over this...my poor baby.

i dont know too much, but im definetly thankful for the help thus far!

 

To check the t-case vacuum switch & the lines, test the vacuum level at the engine manifold port (where the vacuum line plugs in - its on the same fitting as the PCV vacuum line) and then test the vacuum level at the front axle engagement actuator under the battery. They should be nearly the same. If they are not, you likely have an issue with either the line or the switch itself.

The front driveshaft could have a bad u-joint where it connects to the front axle. Depending on your 4wd setup (3-button or 4-button)...the 4-button NV236 system always spins the front axle so a bad u-joint could cause a vibration even in 2HI.

 

thats awesome! thanks for all the help you have provided already so quickly. ill have to check it all out when i get the chance.
do you think the humming is likely a bearing? im kinda hoping it is coz it would make life alot simpler as i have already purchased the hub assembly for the front driver side, i figured its worth a shot.

 

The humming that came from mine last year was the driver side hub bearing. It would do it when the wheel was 'pulling', hence making the noise, in a curve turning slight left. when turning right it was silent and normal.. no noise at low speeds though, say 25mph+ I'd get a humming.

The noise would best be described as the knobby tires on a jacked up truck traveling down the road. I hope that helps, good luck!

 

That's actually the perfect description, or even winter tires on dry pavement. i think that's where i'm gonna start first, i already bought the driver side hub assembly so we're gonna change that out first. Looks straight forward enough, was it difficult when you did it? I looked up some videos and it doesnt seem too bad. Thanks for the help, I just needed that extra input FstFrwd. We're speculating possible tire seperation but I dont think it would be as loud as it is if it was. Slowing down and turning slightly feels like a "popcorn machine" which Im assuming are the wheel bearings im feeling lol.

 

Right, There is actually a thread on here I came across the other day that gives you step-by-step pictures changing out the hub bearing, But I cant seem to find it now? Probably right under my nose.. But It's really not that hard if you can maintain your patients. It takes a 18mm wrench for the three bolts holding the hub/brake assembly to the a-arm's. By far the best tool to remove these bolts would be a "gear wrench". If you dont have a set you can buy a single one, or do like I did and get one from auto zone just for that.
If you can find that thread to view the pictures I was talking about, it would be a big help. Talk to 'swartlkk', I've learned a lot by just reading his posts. He probably knows right where it is.
Another thing, the Hub may end up being 'stuck' and unable to pull off. If this is the case you'll have to get a puller from the auto parts store, and don't forget to get a little pack of anti-sieze to rub on the inside of the new one, just incase the new one ever goes south. It wont be stuck so bad if you have to change it. Good Luck!