The Chrysler K-Car Club

I've been bugged lately with our 86 convertible inside climate control. When in the defrost mode I get a real blast of fan noise going......normal I'm sure. But then I switch to the other settings and get the normal lower fan blast thru the different fan speeds. But every few minutes the "defroster" fan noise cuts in for a few minutes then cycles back off...it's driving me nuts. What's going on here? Mike