on power charger, xps fan comes on every 10 seconds

xps 9570, jackhack96 fixes, ubuntu 18.10.1

when on battery power, it is silent.  when on charger, the fan spins up about every 10-15 seconds for about 3 seconds.  it's quite a nuisance.

unfortunately, pwmconfig finds no fans to control, and my bios does not seem to have a control setting for the fans either.  is there a way to control the fan speed?