well, first off, its and autocorrect of "AdvanceTrac"
"The automatic
ControlTrac four-wheel drive system can be coupled with
AdvanceTrac electronic stability control, which includes
four-wheel electronic traction control. The combination of these two systems is innovative due to a vehicle equipped as such, can continue to move forward with only one wheel having traction.
[7] AdvanceTrac's four-wheel electronic traction control system uses the vehicle's four-wheel four-
sensor four-channel
anti-lock braking system (ABS) and is programmed with additional anti-slip logic to simulate
differential locks via aggressively "
brake locking" either the
front or
rear drive axle differentials. By ABS brake locking the front and rear differentials, up to 100 percent of torque can be sent to any one drive wheel, allowing the vehicle to keep moving, even with two of its drive wheels (one front, one rear) completely off the ground.
[7]"
so it make sense to me that if its software controlled and its sensing the tires slipping over the small bumps it could be aggressively trying to counter it... which could be the noise. My bet is that above a certain speed it follows a different set of rules... and obviously wouldn't do anything when the vehicle is stopped or on smooth roads... or a single bump...