It's probably MG1 or the power-split device (as Toyota labels them). You might also hear the HV electronics while braking, but I would call that a hum. My classic Prius made the same noises, only quieter since it has smaller motors. My FFH is loud enough that my wife, who has really good hearing, thought there was an ambulance in the distance at every intersection the first time she drove the car. If you can borrow another FFH or a Prius, you'll probably hear the same noise. At low speeds (below 5-10mph), you probably will not hear any of these noises; the wheels aren't turning fast enough for the motors to generate significant power, and the regeneration icon will not appear on the battery gauge.
The more prominent whine is the electric motors & PSD under load stopping the car. At higher speeds, especially 40+ when you are near or past the threshold that the engine must turn, MG1 is spinning near its redline while braking in order to generate maximum power to charge the battery. When the motors are under load (and therefore the gears of the PSD also), they can make some noise, and this is probably the whine you hear that starts out at a high pitch and gets lower as you slow down. If you don't hear it change until the engine stops below 48 MPH, I would be convinced it's the combination of MG1 and the PSD. Depending on your hearing and the noise around you, you can also hear the HV system transferring energy to & from the battery pack. I'd describe that as quiet hum. If it's really quiet, it's possible to hear this while accelerating, but it is generally more noticeable during braking. Usually the acceleration current is limited to keep some headroom to start the engine without interrupting your acceleration, while there is no need to start the engine during braking. The electronics hum only changes volume; it doesn't change pitch that I've noticed.
I will admit that I have not researched the FFH drivetrain in detail, however I have yet to see or hear it do anything inconsistent with what I learned about my classic Prius, and they use the same overall design.