Urban vibrations

My Shake is in an urban environment. At night when the noise dimishes, the attached pattern emerges.
The strong frequencies appear to be multiples of 11Hz, with weaker ones halfway between them, i.e. at around 16.5 Hz and 27.5Hz, as well as a lower bound at 5.5Hz.
What could be generating these frequencies?
Are there any sources that list common frequencies and their causes?

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Hello friasco,

Some of these seem repeating in some kind of pattern. Do you know if, near to your location, is there machinery that is activated periodically, especially at night?

They resemble similar spikes and spectrogram views that I have seen for Shakes close to larger buildings, or industrial estates. In the end, even a single factory could produce such a signal if it is close to your Shake.

Possibly, other users on this forum may be able to help with their experience?