Why can you hear a humming sound inside your head?
When you are alone, in a room with no external influences, you can hear a humming sound which appears to be coming from inside your head, or somewhere internal. What’s the source of this droning sound?
There are two possibilities with this; noises you can hear that come out of nowhere could be tinnitus, which is caused when you have damage to your hearing system.
The damage removes the input of certain parts of the hearing spectrum, (you hear across a range of frequencies).
In the same way as some people get "phantom pain" in an amputated limb, the brain 'invents' some sound to make up for a lack of input from a part of the hearing system.
Tinnitus is normally a high pitched ringing noise, so probably not the cause of a droning sound.
The most likely explanation for a drone is that you have blood vessels in your ears, and these pulsate. In some situations, such as a very quiet room or when you put your head on a pillow and create a sound chamber in your ear, you can hear the sound of blood rushing through those vessels.