2003 Cadillac Escalade Ext whine

Does the whine occur when you are stationary? Bring the speed up, and press the brake hard. It is a hydro-boost type brake booster that relies on P/S pump pressure. If your fluid is low, or pump is becoming faulty, a whine could occur. If it is accelerating/decelerating whine, that only occurs with vehicle speed, you could have some scored front differential side bearings as well.