Q: My macbook fan is constantly running

Resetting the System Management Controller

Apps can affect Mac performance, battery runtime, temperature, and fan activity - Apple Support

It is not a question if your fan is running but at what speed.

If the SMC does not provide a solution, install iStat menus which will provide temperature and fan speed information:

http://bjango.com/mac/istatmenus/

Open Activity Monitor to ALL PROCESES and set CPU to display values from high to low.  This may show what applications may be responsible for the high fan speed.

Do not dismiss the possibility that one of the fans may be faulty.  Run an Apple Hardware Test:

https://support.apple.com/en-us/HT201257

Ciao.