Use an n-channel MOSFET
When the Pi is powered down, the GPIOs are GND, so the currently in use p-channel-MOSFET opens and causes the fan to blow at full speed. Rather use an n-channel-MOSFET instead.
When the Pi is powered down, the GPIOs are GND, so the currently in use p-channel-MOSFET opens and causes the fan to blow at full speed. Rather use an n-channel-MOSFET instead.