1. Your PS3 may have a faulty temperature sensor making it unable to detect the heat of your PS3 and it's default is to set fan speeds to max incase of overheating.
2. Your PS3 may have a faulty fan due to previous dust or heat. A replacement may work.
3. If it's an older PS3 the CPU heats up more for newer games, the newer games use more CPU power causing it to heat up faster and the fan to speed up to prevent overheating.