Hey noiza, I had the some problem with mine (the 180W/ non-kaby lake version) , see this thread:
https://rog.asus.com/forum/showthread.php?90178-GL502VS-Sometimes-one-of-the-fans-spins-up-very-loud...The difference between mine and yours was that mine wasn't just constant all the time, mine would do it randomly for maybe 30 mins at a time, and then the fans would go back to being quiet again. It would do this on and off several times a day.
It hasn't done it at all in about 3 weeks now, so something has happened to fix it, I have no idea what. I'm very afraid of making any changes to the BIOS, or graphics drivers, or anything really, incase it comes back.
There were some things I was doing around the time that it stopped happening which MAY have helped, I took the back cover off several times and disonnected/reconnected the fan on the right hand side (GPU fan), which was the loud one for me, the CPU fan didn't have this problem. Maybe try doing that a few times and see if it goes away?