It sounds like your machine is getting hot quickly and tries to cool itself off by running the fan at full speed.
There might be several reasons.
Air vents get blocked
Sometimes, dust and dirt block the air vents and it can't create enough airflow to cool down the CPU. In a situation like this, take a look at the air vents and make sure they are clean and have good airflow.
Something else is using the CPU
If dust isn’t the problem and you’re not doing anything CPU intensive, but your fan is still running at high speed, some other program on your computer may be accessing the CPU. It could be malware. Open Task Manager and check what program is accessing the CPU. If you get something unusual turn off the program and if possible uninstall it.
Most laptops have a temperature sensor that controls the fan speed. If the temperature sensor got damaged, your laptop will unable to control the fan speed.
Hope this will help you.