ABS is what I suspected, even before you mentioned the sound ... :thinking:
Do you have the traction control / etc. ?
The 4WD engagement wiring doesn't interact directly with the ABS circuits, but the traction control stuff does. The 4WD circuit interacts with the Skid Control ECU (which uses the ABS to manage the wheels).
I'm wondering if there's a hydraulic fault (like a pinched line) or an electrical fault that's causing the ABS to go nuts ... If it's the ABS, I'd just about bet the Skid Control ECU is involved somehow.