Birds in the wheel well!

I drive a 1997 Nissan pathfinder, 5spd, 4.2L, 150K miles. I have a chirping coming from the front end of my car. The chirping, however, has the power of a tree full of birds! I have had both front wheels rebuilt 3 times, bearings repacked twice, dust shields changed, rotors resurfaced. The noise is almost always present when driving the car first thing in the morning. The noise doesn't begin until we are in motion, and I back off of the accelerator. The chirping will continue if the car is gliding in neutral; feet on or off the clutch. I have had three mechanics tell me that my car gives them nightmares!
What could be the cause of this elusive noise?
--Karrie; San Diego, CA