Heat adjuster in front clicks when sliding from cold to hot

When I put the heater on, it only blew heat in the back, blowing cold in the front.
i could hear a clicking sound as I slid the temp adjuster from cold to warm.
when on cold - no clicking, as soon as the sliding adjuster passed the midway point- more clicking.
after a while the clicking stopped and hot air came out.
i tried it again an hour later, the clicking was there again, and hot air was only blowing in the back of the van, cold air in the front, slide the temperature adjuster to cold and the clicking stopped again.
How can I fix?