XPS 15 9550 fan turns on when plugged in

This is my second XPS 15 9550, the first one had a similar issue where the CPU fan would turn on and run full speed when plugged in (up until about 80% then it would shut off). On this unit, when it is plugged in, another fan turns on but it's much quieter. I'm guessing it's a GPU fan? Does the CPU fan only have 1 speed? When it is unplugged the laptop the fan turns off.

I have all current video drivers and BIOS and all power settings are set to passive. Is this a hardware defect and should I return it for a 3rd unit?