It sounds to me like you are hearing the electric brake booster. Traditional car engines use vacuum from the engine to boost the brakes, but on a hybrid that does not work when the engine is off. Instead, an electric pump is used.
If you open the hood, and stand in front of it (with the engine off) while someone repeatedly presses and releases the brake pedal, you will be able to hear exactly where the sound is coming from, and it should be the brake booster pump.
(Note: this is based on my 2017 Highlander Hybrid and a friend’s second generation Prius. Since the 2008 Highlander hybrid falls between those two vehicles, I see no reason that it should be any different.)