Why do power lines buzz? When near high tension power lines, particularly after a good rain, the lines themselves emit a buzzing noise. A similar noise can be heard coming out of the electric meters attached to my apartment.
I've heard before that this is supposedly from the 60Hz AC current that's running through the lines -- namely, that the buzz is the same 60Hz which is in the lines.
I'm skeptical of this though for a couple of reasons:


*

*I don't see any reason the change in electricity would somehow be audible.

*The noise subjectively sounds relatively high pitch. 60Hz would sound extremely low pitched -- it's near the base of human hearing of 20Hz (typical).


What is the actual cause of that buzzing?
EDIT: I just spent some time playing with a tone generator and the noise I hear from these things sounds closest to 120Hz using a square or triangle wave. (Oddly, not a sine wave, as I would have expected) Perhaps that helps?
 A: Well, it's only a Wikipedia reference, but it says for high voltage power lines, it can be corona discharge. For transformers or other objects containing coils, it can be magnetic. Seems to me, for things carrying AC, it's pretty hard to make them not hum.
If you're wondering how it sounds, that page has some recordings, but you might have to double the frequency (go up one octave), because for a 60hz signal, the voltage and/or current reaches a maximum 120 times per second.
A: Nearly everyone here has posited magnetostriction as the cause for the hum. This is most certainly the case for transformers which have large amounts of ferrous material put in laminated sheets to form the core, but is a doubtful explanation in the case of power lines. Power lines don't have nearly enough magnetic material to produce large enough deformation to give rise to a loud hum that a passer-by may notice. The more likely cause, especially that it is mentioned the hum is noticed particularly after the passage of a train, is the vortex-shedding.
Wind flow over the wire at a sufficiently high velocity, generates a series of vortices that are periodically shed forming the so-called von-Karman vortex street in the aft of the bluff body. Now this phenomenon maintains fantastic periodicity for a reasonable range of Reynolds numbers. This aeolian effect is called Conductor gallop for low frequencies and flutter for high frequencies, which corresponds to the "singing" of power lines.
A: The humming you hear around all things electrical is 120hz, because an imperfect 60hz sine wave has strong harmonics at 120Hz which you will likely hear over 60Hz because of the frequencies our ears pick up best.
The humming you hear from power supplies, transformers, power meters, high-voltage distribution boxes (which have coils and such inside), etc etc is because the magnetic field in transformer coils is a physical force acting upon ferrous metals (this is how speakers work).  Even though you might see a coil consisting of varnished wire that is glued down or epoxied really tight, the magnetic force is still tugging on these wires ever so slightly to great vibration.   It doesn't have to be the coil wire itself either, it could be any metal object around the coil.  The force is there and it's pushing and pulling on that metal back and forth at 120 times a second.
For high-voltage lines outside on poles, it's a different story.   That is, if you aren't around any transformers like you see in those big distribution plots.    What you are hearing is not corona discharge (as that is mostly silent unless when you get total breakdown you will hear and see arcing).   After rain, or when moisture levels in the air raise you get condensation developing on the ceramic insulators that hold up the cables.   You will notice these are shaped oddly like little half-domes so as to make it harder for a stream of water to make a connection between the live line and ground (or another phase).  They aren't perfect though, and when rain or moisture develops on them, it can create shorter little paths for the current to travel.   What you are hearing is tiny little bursts of water boiling off the insulators.
A: The reason for this is something known as "magnetostriction", which is strain induced in a magnetic lattice due to the magnetic nature of a material.  There is a pretty good explanation of the buzzing noises in transformers in the article  Magnetostriction (aka: Why Transformers Hum), but the summary is that the ferromagnetic domains in the transformer core are subjected to the 60Hz (in the US) oscillations of the magnetic field due to the AC current.  Because of the electrical cycle, there are two impacts on the core per AC oscillation, so the strain is changing at 60 Hz, which produces the 120 Hz noise that we can identify a "great B" note.
A: As you can infer from the answers here, there are at least two correct (and different) answers.
A magnetic field exists around an object carrying current. 
This field interacts with the earth's magnetic field to exert a force than acts on the current carrying wire. There is some movement of the wire caused by the magnetic field surrounding it.High tension wires are Aluminum clad Steel) 
I used to work at a 50Kw AM radio station that had a large audio transformer in the transmitter (the transmitter was "plate modulated"). The transformer acted as a Lo-Fi speaker due the transformer winding movement at audio frequencies (the modulating signal).
In the same way, motor windings will make a 60 Hz, plus many harmonics (multiples) of sound when powered with a cheap (square wave) inverter. This is because the square wave contains high harmonic energy (multiples of 60Hz. Loose windings, more noise.
The second way power lines hum is electrostatic, related more to voltage than current (as the magnetic effects described above are).
The lines become an "electrostatic speaker" based of an electric, instead of magnetic field.
(see electrostatic speaker article for explanation)
A: This is the only explanation I can think off : Lorentz force due to earth's magnetic field
Let's do a rough estimation:
Let's say we have some power line at 1000A (should be ok for 100mm^2 wire), 200 meters between towers.
F=200 meters * 1000A * 40 microteslas (some average Earth's magnetic field) = 8 newtons (Not looking at angle at the moment, depend on position & location)
While 8 newtons itself is not much to significantly move heavy wires at 50/60 hertz (this sample wire would weight ~180kg), if line have any mechanical resonance at multiple of 50/60 hertz, vibration will amplify and will be significant to make audible sound.
