2001 QX4 AWD/4x4 issue

[firemed55]

I noticed the other day that when my car is in gear and either AWD or 4x4 drive is on, a humming noise comes from the bottom of my car. If I turn of the AWD/4x4 the sound goes away. Also, if it is on and I put the car in park it goes away. I have heat shields that are loose but this is not a rattle. It is a hum. Almost sounds like a low setting on a fan but with a little more volume. Not sure what it could be. 4x4 service? Low fluids? Any advice would be great. Thanks.

[NY02PathyLE]

I seem to have the same problem....did you ever figure it out?

[thaomyt]

i have the same issue on my 01 QX4.

[SixGuns]

I may be wrong, but my first guess would be load on the gears and you're hearing the resonation of the transfer case with pent up energy waiting to be released...never really listened to my Pathy in 4x, just 'cause I'm usually wailing on the throttle with my stereo way up and my skid plates screaming on rocks....but were I to just listen to it, I'd probably find the same "hum"...I'll check it out tomorrow and let you know if my '04 does the same thing...Unless your transfer case/front dif is bone dry, in which case, there would understandibly be a hum...have you checked the fluid levels in everything?

[NY02PathyLE]

I've done a bit of research....

Seems there is an electric transfer motor on the transfer case that gets turned on ONLY when you shift the transfer case into anything other than 2WD. That motor turns a sub-oil pump in the transfer case that circulates fluid to libricate things when you are in gear and idling, or stationary or at low speed. At normal driving speeds the main pump does the job, but at low speeds or in reverse there is no lubrication/circulation - the motor then does it's job.

If that motor is going/gone, that could be the cause. I'm not sure if this would cause an SES light or anything, but it would be realtively easy to test - just crawl under the truck and disconnect the motor from the harness. Start up the truck, flip the All-Mode switch to Auto and if the noise goes away you have your answer.

[Adnan]

yupp mine does that too its fine in AUTO but HI and LOjerks the truck around whenever i give it some gas

i got the dealer to look at it few weeks back and everything seemed fine the fluids were all topped up and could last me another 6 months

[NY02PathyLE]

I did some highly technical diagnostics work to figure out what was going on - I pulled the fuse for the transfer motor (in the engine compartment) and when I flipped the switch to AUTO, 4Hi or 4LO the noise is nonexistent. Put the fuse back in and the noise is present. I did get a 4x4 light on the dash when I started her up without the fuse in, but it went away after I reinstalled the fuse. Definitely the electric transfer motor for me. $245 from the dealer....thank god my used car warranty will cover it.