What's causing a spooky hum to fill parts of San Francisco?

The Golden Gate Bridge hums now.

San Francisco residents shared recordings of the sound on social media last week and over the weekend as high winds swept through the Bay Area. One Twitter user described the sound as a "three-tone dissonant soundtrack," and others reported hearing it from as far away as 3 miles (4.8 kilometers).

Related: What's that noise? 11 strange and mysterious sounds on Earth and beyond

SF311, which provides non-emergency services and information for San Francisco residents, tweeted that slats installed along the bridge's bike path were responsible for the noise, citing the Golden Gate Bridge sergeant, an officer with the California Highway Patrol.

The local National Weather Service office tweeted that west-northwest winds of 30 to 35 mph (48 to 56 km/h) had been reported in the area, with gusts up to 43 mph (67 km/h). Even higher winds were reported in nearby counties. 

KQED reported that the slats were added as part of a "necessary" long-term effort to make the 83-year-old bridge more aerodynamic and insure its long-term structural integrity. Wind tunnel models of the retrofit had already shown the slats might hum under certain conditions.

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The effect has turned the bridge into a giant wind instrument — maybe the world's largest — as one Twitter user noted.

The University of New South Wales (UNSW) in Australia offers a helpful online guide to sounds like this. In general, wind instruments work by turning the energy of moving air into a standing wave — a pattern of vibration moving back and forth in the instrument. The instrument's cavity determines the frequency — and thus the sound — of that standing wave. And the instrument itself amplifies the wave into a sound loud enough to hear.

The details of this process vary widely from ancient flutes to oboes to a glass bottle someone might blow over. Materials, tuning and airflow determine what sounds get produced. 

But whatever particular dynamics of the bridge slats that have produced this eerie sound, it looks like San Fransiscans will have to get used to it on windy days. Wooooooooooooooooooooooooooooo.

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Originally published on Live Science.

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