Randomly occurring noise in engine compartment

While driving a strange noise sounding like a revved up electric motor (kind of like when you turn on the windshield washers when there is no solution in the reservoir) kicks in. This occurs very randomly. I put the transmission in N and it remains weather I accelerate then or let the engine drop to idle speeds (while coasting at speed). The only way it stops is if I slow to a stop. I do not have to turn off the engine. It happens going up hills, going down hills, or even pulling up to a stop to park (but then it stops when the wheels stop moving).

I am confused and my dealership has never heard of this before.

My best guess is that it is the thermostatically triggered fan on the radiator. Perhaps the motor is kicking in and not spinning the fan sometimes, or something like that. Or it could be something to do with the power train, but there are not engine lights or noticeable impacts on driving or power or anything.