Cause for Power Transmission Tower "Breathing" OK, this question is not your usual one: Last night while hiking solo from the mountains back to my car at the mountain/desert interface (Lone Pine, CA), I had a rather bizarre -- and downright spooky -- experience.
The last half-mile of my return was in total darkness, which wasn't a problem since I had prepared for that unintended possibility. Partway into that last portion of the hike, with my LED flashlight piercing the remote, desert darkness, I began hearing a slow, distinct and loud "breathing" sound, both inhaling and exhaling, which became more pronounced the closer I got to my car.  The source seemed to be perhaps a couple hundred feet away. I'll skip the whole middle part which had a Blair Witch vibe to it -- at one point I froze in my tracks in the darkness for a minute or two, pondering what to do. Ultimately I realized the sound was coming from a large, steel, high-voltage power transmission tower. I had noticed earlier that that particular tower had been louder than the others in the daylight, buzzing and cracking, but nothing more than that. 
The loud, rhythmic "breathing" noise began after the sun-baked tower began cooling down, with a modest 5 MPH wind present.  It sounded EXACTLY like an animal slowly "breathing." I'm familiar with the corona discharge effect of such towers. But anyone have any insight into the pitch-black desert "breathing" phenomenon?
 A: If there is an aeolian wave through the wire it might sound like that when it reaches the tower and it gets reflected back. Like a mild shaking for half a second or two and then nothing. Waves travel in the 150 ft/s range, so a 800 ft span will repeat every 5 seconds or such.
A: I think it might be the wind. When changing direction it could have changed how some parts were impinged so these were changing emitted sound.
A: When electricity is transmitted along power lines, standing waves get set up along the line's length. This means that there will be points along the line where the line voltage falls locally to a minimum and other points where the voltage rises to a maximum. 
Depending on the nature of the load impedance that the line is feeding into, it's possible for the position of the voltage "nulls" to shift slightly back and forth along the line with changes in the load impedance and in the amount of power the line is carrying.
The line voltage determines how much electrical leakage the power line experiences. It is that leakage which produces the buzzing and snapping and hissing noise that you hear coming from high-voltage transmission lines. This means that if you are standing near a power line and you happen to be close to a voltage null, the line will be "quiet" in that vicinity- until the position of the null shifts up or down the line, which will then cause the hissing noise to increase.
You may have been witnessing the travel of a voltage null back and forth along the line, which would create the "breathing" effect. 
