In my opinion this is a relay of some sort, a fairly large one at that, more like a contactor really, that activates when you touch the brake pedal.
I would love to see the electrical drawings for this car, does anyone have them?
The mechanical brakes themselves will not engage when you just lightly tap the pedal, so there has to be related to the regeneration system, but I don't get why you'd have a big ass relay/contactor in the system. The brake light relay should be very small and mounted inside a water proof enclosure inside the motor bay, can't hear that for sure.
To me it sounds like it is coming from underneath somewhere towards the rear of the vehicle, it is not really loud enough to bother me but I could do without it. 100% not a brake pump or trapped air, or anything of that nature, that would be a first if someone would be able to hear fluids circulating through steel piping under the carriage.