Sounds like a sump pump

[GooseHunter]

I have a 2006 Z71 Tahoe with 150K miles. Recently I noticed an odd sound coming from somewhere in the front of the vehicle when I pull away from a stop. With very little throttle input, the noise sounds like an electric sump pump trying to pump fluids. I can hear it and feel it slightly through the steering wheel. It stops after 5 or 6 cycles. It happens once the vehicle warms up and only in two-wheel drive. If I place the selector in Auto-four, it doesn't make the noise (although sometimes the traction control message comes on at low speeds). I've checked the power steering pump, changed the selector switch in the dash, and changed the 4x4 controller at the transfer case. Any ideas are welcome.