Not necessarily. My GPU audibly sings in the KhZ range whenever it comes off idle, a long time before the fans actually start up. It could be electrical noise from the fan drivers and/or motor coils if they're running at low speed, but it's definitely not the sound of air movement. And if you're e.g. processing photos on the GPU, you can hear it starting and stopping, exactly synced to the progress of each photo in the UI.