My '97 F150 radio emits a loud buzzing noise when I am tuned to anything but the strongest AM station when the blower is on. When I select a higher blower speed the buzzing gets louder. When tuned to an FM station all is quiet and perfectly normal. I checked and all of the connections including the antenna seem tight. How can I get this radio to play an AM station, which is not in my backyard, without all of the interference? I am grateful for any assistance.