99 Blazer 4x4 has constant noise-humming that occurs non-stop and increases and decreases as speed fluctuates. There is no clicking,popping or grinding just hum. Doesn't change whether 4x4 engaged or not. Sounds like it is coming from rearend but not 100% sure. Tried 4-lo or4-hi and 2wd and noise is still present. Same noise even if put vehicle in N to coast. Any help would be great. Have had the truck a couple months and the noise has been constant. The noise is loud enough that it is very concerning.

 

check that your dust shields arent rubbing your brakes, you have to take your brakes off to check for rub marks.

 

Check your front wheel hub bearings.Withmy '01 Jimmy, theymay feel "solid" but once you remove them, they feel pretty rough. I've replaced both fronts after 85k miles.

 

If they meaning anything, were rubbing or a bad bearing ,would it not get warm ? I have walked around vehicle after long ride and touched many diffferent components and nothing is unreasonably warm or hot.

 

Does it make any difference if you turn left or right?

 

no. the sound slowly increases as speed increases and is loudest at 40km but still loud at any speed. Have tried turning left and right, forward or reverse and also in 4lo and 4hi. Noise stays the same according to actual speed driven. Thought it would multiply in 4lo but it didn't.

 

The first thing that I would check into is the oil level in the rear differential. I would actually just pull the rear cover and have a look inside. Clean things up while you had the rear cover off and reinstall the rear cover with a new gasket then fill it with some fresh 80w90 GL2 gear oil (or 75w90 GL2 synthetic gear oil).

Next, maybe take some time and remove the rear driveshaft. Once you have it removed, feel for rough spots in the u-joints. If you find one that feels rough, replace it. The rear u-joints are held in with injected nylon and will require heating while pressing out the old cups. If you are going to replace one, you may want to consider replacing both of them to get it out of the way. Choose replacement joints that have grease zerks so you can grease the replacement joints as part of routine maintenance to extend their service life. Another hint when trying to install the new cups; use a flat file to clean up the inside surfaces where the replacement cups' c-clip retainers will contact. Get these surfaces back to nice, clean metal and ensure that they are flat.

If that checks out, it is possible that it is something internal to the rear axle (pinion bearings, carrier bearings, or outside axle bearings).

 

checked rear axle lub and it's fine. also took back wheels off and checked axle and driveshaft play. seemed fine. I think it is either rearend internal or transfercase.

 

Found the problem or should I say problems. After 2 garages we found that front left wheel bearing was ready to fall off and the pinion bearing in rearend was shot. The part that pisses me off is that the vehicle was safetied only 4000km's ago. Drives nice now but cost $700.

 

Tonight I experienced the same problem of "humming" coming from the right front....we have had alot of snow in the last few days and now it's -10 degrees so I've been using 4 wd hi, today the roads were clear so I switched back to 2 wd and I now have a humming noise, if I switch to 4 wd hi it goes away, 4 wd lo does not appear to work the light only flashes, I'm thinking the vacumn switch maybe bad....fluid in case is good, no play in wheel (like a wheel hub) My thought is the 4wd is not fully disengaging any thoughts??? AMXAL