1995 Ford Probe Serpentine Belt

Engine problem
1995 Ford Probe 110000 miles

My car began to make a squeeling/screeching noise last spring. It made this noise at all times, when the AC or defrost was on and when they were off. Then, I had the serpentine belts replaced in June, and the noise stopped. The mechanic forewarned me that the belt may loosen after a few weeks, and the squeeling would return. Sure enough, after a month, the squeeling returned. I took it back to the mechanic and he tightened the belt. For the last 6 months, I have not had a problem. Now, that the weather has turned colder, I am having the same problem. The car squeels and screeches at all times; however, I did notice that the noise is much more profound when the defrost or AC is running. I am wondering if there's another problem with the air compressor or something else. Any advice would be much appreciated. Thanks!