Prometheus

Hi, I have a 1990 GT all stock and with an automatic transmission. When the outside temperature is around 60 or so and the car is somewhat cold, there is a odd noise when the transmission shifts from 1st to 2nd or 2nd to 3rd, there is a belt like noise. I have had my tranny checked fluid and filter changed. The tranny was said to be in good condition. The belt has some small surface cracks but looks like it is in ok shape. When the noise happens I can feel a vibration in athe gas pedal. Any ideas or Info welcome. Thanks!