When you go into autostick or turn ESC off you basically disable 2WD and force it into all-time AWD. We have long suspected the sound comes from a partially engaged or partially disengaged rear drive since the engagement of the rear axles can be varied in the differential from 0 to 50% (of engine power, ie in snow mode the split is supposed to be 60% front 40% rear), and the PTU (power take-off) is just on/off. So, it might be for instance that the PTU is engaged in the front (dog clutch, all or nothing) but then perhaps the variable clutch in the RDM (Rear Drive Module / Differential) is at low or no engagement, in this scenario the unloaded spinning driveshaft could be producing harmonic vibrations or perhaps free spinning unloaded gear assemblies in the differential itself could be doing the same thing. This whole theory could work in reverse too, that possibly the RDM is at least partially engaging its variable clutch causing the driveshaft to spin but say its not actually "hooked up" (front PTU not engaged) then once again a driveshaft or gear assembly spinning without actual load on it could cause the harmonic vibrations. Yet another possibility is that the variable clutch itself simply makes the slight vibration/noise when its say 50% or less engaged, as a result of letting slip some of the power being sent rearwards by the driveshaft.
Any way you dice it, it seems to be a minor side effect of a variable disconnecting rear drive system. The conditions that cause it vary as the system itself (in normal auto mode) uses a combination of factors to decide when it is appropriate to switch to 2WD to try to save you some gas. It tries hard to predict in advance when AWD/4WD is going to be needed and it has to because of the usage of a dog clutch up front, to smoothly engage that requires the driveshaft to be matched to the speed of the PTU's input otherwise you will get a nasty clunk. As it is you probably notice a minor "thunk" when you surprise it by throwing it into autostick or 4x4 while driving at speed, it has no way to predict that so as fast as it can it has to engage the rear clutch to put some speed into the driveshaft and slam the teeth of the dog clutch into place so you can have AWD in a little less than half a second.
Its a high speed system that only dips its toes into 2WD when all the conditions are just right and even then, it wants to be ready to switch back to AWD at a split-second's notice. The minor amount of humming and clicking and such that goes with it is no big deal to me, it just means the system is doing its best to give be the best possible mileage and traction performance.
Yes, there is harm is turning ESC off. So much so that pressing the button momentarily actually is only what they call "Partial off" (in the four wheel drive models). Turning it completely off requires some more specific steps and can only be done at a stop, and event THAT turns itself back on partially once you go over 15 MPH again. The ESC gives you traction control assist, stability assist, panic braking assist, etc and all of these things make the vehicle safer to drive. Turning these features off as a matter of regular habit is a foolish thing to do. I can say that now because I'm old and wise enough... I used to have a hot-rod honda in which it was my second nature to hop it, twist the key and pop the traction control button to OFF. I didn't like the car cutting the power when I dumped the clutch or drove aggressively. Even then, young and dumb as I was, I was smart enough to turn traction control back on during a long highway cruise.
There is no legitimate argument for regularily turning ESC off (to avoid the humming sound) when the information available suggests
1) That the sound is normal operating noises that are nothing to worry about in the first place, and
2) That you would be putting the safety of yourself, your passengers, and others around you at increased risk of injury or death, and
3) You can accomplish the same GD thing by just popping the 4x4 button...
Seriously, this paranoia over barely detectable humming sounds has got to stop. With all the clunks and bangs and thuds, clicks, humms, pops, snaps, and whirrs that go with driving I am astounded that anyone picks up on this tiny background sound at all.
The safety concerns don't bother me. As I said above, I drove for 25+ years without ESC and never needed it.
As for the "barely detectable" humming noise, it reminds me of the noise the tranny in my '86 Buick made just before it failed. I've had wheel bearings go out that didn't sound as bad. Now that my dealer has raised the speed at which the noise occurs it is masked by the road noise so it doesn't seem quite as loud. If I was running snow tires or A/T tires I probably wouldn't hear it at all. Or as the car guys used to advise, maybe I should just play the radio louder. I'm thinking if I tune in a football game and really crank up the sound, the crowd noise would probably cover it. All fixed.
Seriously, there are two things that have me worried about this vehicle:
1) This grinding noise that shouldn't be there, and 2) The oil consumption issue.
The oil consumption isn't alarming. Its about the same as my old Patriot at 300,000 miles. It was acceptable for an engine that old. I fully realize an engine uses oil up to 10,000 miles, but I'm past 9,000 so I'm getting concerned. I had my oil changed yesterday and between last night and today I've been 400 miles and I'm showing down 1/2 quart. I didn't check to see if my dealer shorted me, but I know where the oil level is now. I'll check again after the weekend and if its down another 1/2 quart I'll know I've got a problem.
These problems may be "normal" for what they make, but its not "normal" for vehicles I want to own. I hope this vehicle doesn't turn into a bad experience, but I'm concerned. I dumped my new '93 Cherokee after 6 months of owning it because the clutch wasn't right. It was in 4 times in those 6 months and I went to a GM dealer and for 3 more payments I traded for a vehicle that went 140,000 trouble free miles. I kept on buying GM after that until they stopped making Pontiacs and my dealer switched to Chrysler products. I'm not afraid of switching back to GM if this doesn't improve.
That said, I'm not upset with my dealer. They have to work with what they've got. However, I'm wishing I'd bought a year-old Patriot for about the same price as my Compass. The rebates made the Compass look like a bargain. Now I'm not so sure.