Blower will not blow on vent

Hi, I have a 1999 Police Package Tahoe (5.7L 350) and whenever I turn my blower vents to high, or medium, I hear a "valve close" and my blower stops blowing with vents and air goes to the side window defog/defrost vents. Any ideas on what this could be? Also, when my blower is set to "high" it is sometimes more powerful than other times. Oh , and forgot one more, when I have it set to floor, it blows out the mid level vents also. Please email me if you have any suggestions that don't mean dropping tons of dinero... Thanks