Sounds to me like you have a computer that’s running fairly hot.
Most computers have a temperature sensor and a variable-speed fan. As your machine runs, it keeps an eye on how warm it is and decides at what speed to run the fan.
It sounds like your machine is getting hot quickly and tries to cool itself off by running the fan at full speed. That cools it down somewhat so the fan can be slowed down, but then it heats up again and it’s full speed (fan) ahead!
The real question here isn’t why your fan runs so much, but why is your computer getting so hot? There are several reasons why this might be happening.