Auto 4x4 noise?

[Tonyv__]

So between my 2015 yukon Denali and my prior 2000 Tahoe I notice when I have the truck in auto mode. I hear like a weird hum sound from the front when the truck is moving. It’s not an electrical hum. Just something in the distance if that makes sense? The sound goes away when I put it back in 2wd. Am I crazy? Is this common? Or is my truck broken lol

 

[PatDTN]

Just a guess but the front axle probably stays locked in when in auto mode so it can kick in and out quickly. In 2wd it can unlock so the front diff doesn't turn and probably nets you a mpg.

 

[bill1013]

I had an 1984 Ford Bronco II 4X4 and it was a real beast. I could go up hills that other 4X4s couldn't. The only hassle was to put it in 4WD I had to stop, put on the hand brake, put the trans in neutral, get out and lock the front hubs, then get back in and go. When I got my 1999 Tahoe LT 4X4 (5.7L) it had electric switching, buttons for 2WD, 4 Hi, 4Lo & 2Hi (if I'm not mistaken). I was told by the dealership that you could "change gears on the fly" meaning you didn't have to stop and change modes. I tried it once and it seemed to go into gear okay, but taking it out seemed to be a different story. You know how sensitive the steering is in 4WD well when I took it out of 4WD on the fly the steering stayed the same. Which can be scary when you get up to speed and you just touch the wheel and it over steers. So I ended up going back to the old way of shifting sort of. I would stop, engage the hand brake, put it in neutral, select the mode I wanted to drive in, then go. And reverse the process to take it out of 4WD. When it was stopped and I put it in gear I could feel a little "clunk" and hear the gears engage. So even though they say you can do it while moving I learned it's better to stop. My friend's husband was working a Chevy dealership and he helped me with some minors problems and he said that even though Chevy says you can do it "on the fly" it's better to stop and do it. My logic was...do you shift a manual trans without pushing in the clutch pedal? I wouldn't and the 4WD gears don't have a clutch so I figure they grind and possibly stick when trying to switch back to 2WD. Stopping and switching took a few moments, but I never had any trouble with my 4WD again. Also, changing the oil in the transfer case is a definite must. I found the heavier oil (90 WT.) needs to be changed around every third engine oil change.

 

[Bill 1960]

In auto mode some components are spinning that are stationary in 2WD, namely the front driveshaft and transfer case front output, and the front ring and pinion gears. That’s the source of the noise.

And for those unaware of it, the auto transfer case DOES have a clutch. A computer operated clutch is how it engages the front axle when the rear needs assistance. There’s no reason you shouldn’t use it on the fly, exactly as it was designed to operate.

 

[PatDTN]

I had one of the first Broncos IIs. It came with auto locking hubs. As long as i had the hubs locked by starting off in 4wd and shifting out i could shift in and out of 4wd on the fly. I did discover that the extra drag from having the hubs locked in caused the front to slide on slick surfaces if i didn't use 4wd.

That ability to auto lock stunk when I got bogged down and needed to rock to get out. The back tires made a full rotation before the front hubs unlocked and relocked every time I reversed direction.

Back to the OP that's what makes me think it's your front hubs being locked and causing the front diff to make noise.

 

[wjburken]

Back to the OP that's what makes me think it's your front hubs being locked and causing the front diff to make noise.

Just for clarity, there are no locking hubs on these vehicles. The CV shafts are always engaged and spin with the front wheels. The “locking” happens at the front differential and in the transfer case.

The one thing that I’m not 100% sure of is if when the vehicle is in Auto4WD, if the front differential locks in and then the transfer case just engages the front prop shaft as needed or if the front differential and the transfer case engage simultaneously.

 

[PatDTN]

Probably why my Tahoe works so much better than the Bronco.

When I bought my used and abused 2009 Tahoe I tried using the auto mode. I could feel things binding up and doing bad things. It had 3 different tires in various states of wear and circumference.

 

[wjburken]

Probably why my Tahoe works so much better than the Bronco.

When I bought my used and abused 2009 Tahoe I tried using the auto mode. I could feel things binding up and doing bad things. It had 3 different tires in various states of wear and circumference.

Yeah, different size tires can really mess with the 4WD driveline. It’s even more of an issue on the Denali’s with AWD. Can be a challenge if you blow a tire and trying to find a replacement with the same amount of wear otherwise you’re buying a full set of new tires.

 

[Tonyv__]

In auto mode some components are spinning that are stationary in 2WD, namely the front driveshaft and transfer case front output, and the front ring and pinion gears. That’s the source of the noise.

And for those unaware of it, the auto transfer case DOES have a clutch. A computer operated clutch is how it engages the front axle when the rear needs assistance. There’s no reason you shouldn’t use it on the fly, exactly as it was designed to operate.

That probably explains what I’m hearing. I generally keep it in 2wd but with the snow we just got. I had it in auto and noticed the same sound I used to hear in my old Tahoe. Just want to make sure it wasn’t something concerning