Hvac Problem

[jackelope]

Hello,
Im having a problem with my temprature control on my 96 crown vic ( without climate control) The problem only occurs when it isnt at the shop ( go figure ) Sometimes i can hear a buzzing sound coming from what sounds like the center of the dash behind the hvac controls but everything still works, and other times the temp will go from heat to ac and vice versa without doing anything. i replaced the main control panel ( Cause the old one was sticking anyway) but this had no effect. the vents and the fan are always working proberly, its just the temp of the air. The buzzing noise sounds like it is a smal motor with a short. Any advice would be greatly appreciated.
Thanks
Dan