2000 Chevy Blazer LS 4x4 Noise

I own a 2000 Chevy Blazer LS 4x 4 2 door with the 4.3 V6 automatic with air. Lately in the morning when I start to drive it I hear a clicking and small grinding noise that seems to be coming from the right front tire/right front engine compartment area (also sometimes sound like a baseball card that was stuck in the spoke of a bicycle rim). It gets louder when I drive faster and dissappers when the car comes to a complete stop. The only thing that seems to correct this is when I shift it into 4 wheel drive and then back into 2 wheel drive. I have had the car to the dealership 4 times now and they cannot seem to locate the problem (the car only has 44,000 miles on it and is still under warrenty). They have told me they have checked the brakes and abs system along with the 4 wheel drive system and all look good (although right now I am questioning that). Any ideas as to what may be causing this as right now I am at a total loss. There are no modifications made to this car, and the most recent work done to this was a new serpintine belt. Thanks for your help.

Here's one to try: Pull the vac line off the firewall mounted solenoid that goes to the 4wd actuator and plug it - if that releases the front partially engaged actuator then that solenoid has an internal leak and is partially engaging the front axle coupler thus the noise. Replace solenoid if so. dealer only and dealer usually needs the # so simply unclip it and read it off.