Invention through annoyance

Here’s the problem – my neighbor’s HVAC closet is doing this, and it’s preventing me from sleeping:

Audacity spectrogram with primary component around 55 Hz
This is the highlighted section

I’ve submitted a maintenance request, sure, but more data more better, right? There are three states here: not buzzing, buzzing, and buzzing without the higher component. 55 Hz primary, 220 Hz higher component. FFTs away!

I was hoping this would be the part where I link to the code, but boy howdy is it harder than I anticipated to get a microcontroller to do this.