The change in the speed of the fan is normal; it is a variable speed fan. The power supply used in the Power Macintosh 7200/7300/7500/7600 enclosure contains a fan speed control circuit. The circuit is designed to monitor the temperature of two critical heatsinks used in the power supply. As the temperature of those heatsinks rises and lowers, the speed of the fan is adjusted to compensate for the change in temperature.