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2004 ford sport trac- loud squealling noise

Recently my 2004 Ford Sport Trac started making very loud squealling noise from under the hood. The vehicle has approx 78,000 miles. When vehicle first acquired this noise it happened when it was first started, now it does it all the time. My first thought was the fan belt. I sprayed some lubricant on the belt and the noise stopped temporarily. I took it to a local mech I deal with on a regular basis. When I returned to pick the car up he wanted to show me something. He replaced the belt but showed me the belt that came off. It looked brand new. He thought I had already replaced the belt, but I had not. He also showed me how he could spray lubricant on the belt and it stopped temportarily. The sound seems to come from the bottom pulley. He checked all of the pulleys and the belt tension after putting the new belt on. I can still spray lubricant on and it stops temp. He said he could not find a cause. I did notice it gets really loud with the heater/ac fan running. The sound is constant but gets louder with the fan on. Any suggestions?
Thanks