Scope the signal wire from the PCM to the fan ECU when the fault occurs to see if the fault is a command issue or internal fan ECU fault.

Fan control unit is integral in the large Fan motor housing.

These can spool up due to loss of engine comm's, faulty fan controller, water in engine fuse box etc....

Scoping the pwm from the engine will tell you whether the fan is being asked to run or has declared it's independence & is doing what it has a whim to do with no external input.