The change in the speed of the fan that you are noticing is normal. The power supply used in the Power Macintosh 7200/7500/7600 series computers' enclosures contains a fan speed control circuit. The circuit is designed to monitor the temperature inside of the power supply and adjust the speed of the fan as the temperature of rises and lowers to compensate for the temperature change.
This fan speed control feature is not unique to this particular power supply. A similar fan speed control circuit is used in other desktop Macintosh computers as well.