When your Mac book pro fan is loud without any dependency in running application, you should have a doubt. Usually the fan could be noisy when your Mac book needs to cool system's heat. But if that is not the case and fan is working loud, it might be caused by some kind of complex conditions triggering that in SMC(System Management Control) associated with temperature sensor in Mac book pro.
We can fix that situation by reset that complex situation stored in SMC(System Management Control) as following:
- Apple menu → Shut Down
- Press Shift-Control-Option on the left side of the built-in keyboard + the power button(Touch ID) at the same time. Hold these keys and the power button for 10 seconds.
- Release all keys.
- Press the power button again to turn on your Mac
You can find more information triggering such an unexpected behaviors at https://support.apple.com/en-us/HT201295