That sound is definately not normal. I have had 3 fans replaced in 4 years (2015 MS P90D) 3 under warranty 1 under extended warranty. - I feel that qualifies my as an expert.
ABSOLUTELY normal. Just gets amplified by the phone mic.
It´s the bigger cooling fans which are shutting down after the car is parked.
They rarely come on in normal driving. Mostly on spirited driving on the freeway and during supercharging (and in super high temp climate conditions).