Fan is very loud after turning on, what could be wrong?

[lovecd]

The loud is very loud on my MacBook Pro 15” 2015 model, I did the following tests:

1. Run apple self diagnostics program and returns no errors.
2. Open the back cover, it wasn’t dusty, fan and other components look fine physically.

I have it to use fancontrol app to make it quiet.

Any thoughts what could be wrong?

Thanks,