Looking recently at the National Geographic article about the "hum" detected from the earth. A hum of roughly 4.5 millihertz observed at any point on the earths mantle.
Possible sources for the hum mentioned in the article were waves and wind. In the following I offer a new original( at the time of writing) and alternative explanation based on the rotation of the earths liquid inner core as it rubs against the harder more brittle outer mantle. And produces the hum, much as a finger rubbing on a glass rim produces a sound.
If one studies the system of running a finger around a glass rim. A resonant tone is produced by the friction of your finger against the glass rim as it goes around the rim. As you rub your finger on the rim your finger alternatively sticks and slides in very rapid succession. Which produces a vibration in the glass. The generally accepted theory is that it is the vibrating molecules in the glass which resonate at a certain frequency in unison at one frequency. Inducing sound waves of a similar frequency in the surrounding air. This can be altered by adjusting the water level in the glass. This increases the mass of the system, reduces the resonant vibration of the glass molecules and lowers the resonant tone. What is noticeable also from personal attempts is that there is also a superimposed overtone that changes in pitch (as well as volume) as one moves the finger faster and slower around the rim. The faster one moves the finger, the higher the pitch produced.
Based upon these generally accepted principles one can then apply it to the Earth as a whole. The Earth consists of a inner liquid core which is rotating around the equatorial circumference inside the Earth. This in turn rubs against the harder more brittle mantle. Which produces, like the finger on the glass, a resonant frequency that is then picked up by sensors at the sea floor. As the liquid core/mantle is one whole defined system in the same way as the finger glass system is, it follows that at any point around the mantle the resonant frequency will be constant. As is observed with both the earths "hum" and the finger/glass system. Generally, as with the ringing glass, the earths hum is roughly constant at any point on the globe.
Quantifying exactly the specific observed 4.5 millihertz frequency is not possible to do for me at this point, not least because so little is known about the exact conditions inside the earths core. But the general rules seem consistent with the observed frequency of the earths hum. The mass of the Earth system is much greater obviously, then the finger glass system. Accounting for, at least in general theoretical terms, the very low observed frequency of about 1 cycle every 222 seconds.
It is also possible to account, at least in part, for the specific "earths hum" frequency being related to the speed of the rotation of the liquid core flow. As I have pointed out earlier that the pitch of the ringing glass changes with the speed of the rubbing finger around the rim. Implying that the very slow rotation of the liquid core relative to the earths mantle (centuries for one complete rotation) would also be consistent with the observed low frequency of the hum.
One point I would like to make here is that currently the generally accepted theory of the movement of the earths liquid core prohibits a rotational motion of the liquid core. Which in turn means the model I present here would not be able to explain the overall steady friction of the liquid core rubbing against the mantle. Which is probably why theorists have not considered this option I present here. Current theory (i.e./Glatzmeir) predicts only localised thermal eddies in the liquid core. However as I have theorised with my paper on the 'Earths Magnetic Field'(available here on this blog site from the table of contents) a rotating liquid core is not only consistent with observations. But can in fact explain the earths magnetic field, direction of polarity, movement and direction of magnetic North Pole and finally the frequent historical geological record of the flipping or reversal of the earths magnetic field. In that the rotating liquid core mantle solid inner core system is acting as a single system rotating dynamo.
