Fan always on when charging

I have a Precision 5520 running ubuntu stock/archlinux. On both OS (kernel 4.4 and 4.14):

- when the laptop is unplugged to power sourcce, the fan is turning on only when temperature is high, I'd say it works as expected

- when the laptop is plugged to power source, even if it's cold and idle, I see fans spinning around 2500 rpm all the time while sensors temperature is ~30. I tried to adjust tlp options but it doesn't change a thing

Support proposed to change motherboard and fan but I think it's not a hardware issue. Anyone else have the same issue or it is working fine?