why macbook pro fan started running as i switch on

1. Reset SMC by following the directions applicable to your MacBook Air here ➔ How to Reset SMC. If that does not solve things then-
2. Reset NVRAM/PRAM by following the directions applicable to your MacBook Air here ➔ How to Reset NVRAM/PRAM. If that does not solve things then-
3. Enter ➔ Safe Mode and see if the screen issue still exists in Safe mode. Log out of Safe Mode and then reboot computer. 

Axel F.

Firstly, is your MacBook running hot? Well your Mac will turn on the fans if it’s hot, of course.

next, do you have Macs Fan Control or any can control app installed? That may be controlling fan speed.