Just tried putting my 99 Blazer LT into 4wd. Seemed OK at low speed but when I go up to and over 20 mph I started to hear a loud hum. I played around switching in and out of 2wd, 4 hi, and auto 4. Sometimes it wouldn't com out of auto 4. I had to stop and try again. Anyone else run into this? I know if I take it in to the dealer they'll hit me with $200 just to tell me what wrong.

Were you operating the truck on dry pavement?

Are all of your tires inflated to the same pressure all around?

Do you have the SAME tires on all four wheels?

Have you recently replaced tires?

It's very important to keep the tires on a 4wd rotated

Thanks for the comments, tire seem good, fairly new. broke down and took it to the dealer. $850 later they replaced the pinion bearings in the front diff. After driving out of the shop and putting 4hi on it still seemed like there was a loud rumble. Went back to the dealer and had the mechanic drive with me, he thought it loud but better and said it was probably just the wet pavement. (didn't really believe that).

Since then been driving in auto4 and 4 hi. Still seems loud when in 4hi. When in auto4 seems like it pulls alot too (every couple of minutes). Also maybe my imagination but it seems like there's a bigger drain on the battery when it's on (headlights dim when coming to a stop).

mechanic mentioned that wrong fuild was in transfer case for auto4 and he replaced with correct?


also seems like lights on the 4wd-selector buttons get 'confused' sometimes they stay on, sometimes they all go out.

would standard "check vacuum hoses" apply hear?
any other hints?

Have your front wheel bearings been checked, very common problem, and they make a lot of humming noise when they are going.

Hold on, I just read your title, is the noise only coming when the 4x4 is on, or is it all the time?

Hi, Just in 4wd above 30-40 mph. Had one front bearing done last year the other the year before.

Yeah that is usually a sign of bad differential bearings, but after it is dissasembled, it has to be put back together properly, set up properly, get them to check the front diff again. Tell them your not satisfied, and the truck did not make that noise originally. Also have they mentioned anything about the transfer case? How is the condition of that unit?

after charging 300 to rip down the front diff , I wasn't sure I wanted them tear that apart. Would the pulling be symptomatic of the front diff?

Hi, Just in 4wd above 30-40 mph. Had one front bearing done last year the other the year before.

So the dealer soaked you for $850 and it didn't fix the problem?!!

Take it back and DEMAND they fix it for free now. This is their problem now. You paid them to replace parts that didn't need replacing.

You might just need to change the diff and transfer case fluid.

Now they're saying all four tires need to be the same. We have a matched set on fron and an matched set on back. Anyone buy this ???

Sorry I never thought of that.

I'd only have a matched set of 4 tires on any 4wd. Every different brand is a little different size wize.

I know that it's a big expense when it comes time to replace them, but what can I say except that's 1 of the joyes of owning a 4 wheel drive.

It's even worst with a posi, then you even want the spare to be the same.

Yes, four exact tires, never two different sizes or you will be putting more stress on the drivetrain.