At face value this probably sounds like a hard drive skipping, but what you're hearing is actually quite the mystery. Let me explain:We've all heard me go on and on about the hidden world of ultrasonic sounds, but INFRAsonic sounds, AKA the sounds and vibrations below the human hearing range, are equally as interesting, but many times more difficult to capture. If you recall from The Hum video, I used a metal box with long tubes that spread out over a 20x20ft area to create a giant geophone of sorts that allowed me to record infrasonic and seismic data. I wasn't too reliant on it as "the hum" is heard by humans, so it's hunting for the wrong frequencies to begin with. But it did get the juices flowing for the sounds underneath.One does not just buy an ultrasonic microphone and plug it into a recorder. The sound capturing process works much more like a vibration analysis task. Which compressions and rarefactions are we hunting for? Up and down? Forward and backward? All 3 dimensions? Fortunately geologists have been doing this for a long time and seismography has come a long way from the roll of paper and the mechanical pen. Most data is captured in their own formats made for this type of measurement, like SAC or MiniSEED, which is a goldmine in itself as seismology requires everyone to share their data to rule out localized noise when trying to figure out what's going on geologically. Unlike the ultrasonic world, there's tons of noise in the infrasonic realm due to how far and spread out low vibrations travel. An enormous amount of noise pollution is also created by infrastructure, but this is actually kind of cool as nearly every single thing you pick up is an absolute mystery. For example, in the image below shows about 24 hours of recording from a seismograph outside in the corner of a tool shed on my farm. Every 15 minutes or so, there's a bunch of activity that is nearly identical. Maybe it's the water heater clicking on? Or the air conditioner pump in my studio routinely pulling in new water? Around 3am, we apparently had a 30 minute long enormous rumble. I was on the porch responding to emails at the time and certainly didn't notice it. I don't even have a theory! Monitoring this data isn't very challenging, but listening to it is more complicated than one would think. You can't just dump a seismograph file into Audacity. Not only are the waveform ranges completely different than a digital audio file, but a LOT of math is required to simulate the waves to read as pascals, or sonic pressure waves. Once this is done, a bit more math is required to build a time series, and finally, after about 2 years of occasionally going on frustrating binges to get an "infrasound microphone" working, I have a workflow! What you're hearing is part of that tool shed recording. You'll hear occasional plonks and rumbles as if I'm adjusting a microphone. Amusingly each one of these sounds typically lasted for 5+ minutes in real time and were, of course, too low for the human ear to pick up. Those high pitched tones are steady rumbles, probably from nearby HVAC units. The routine clicking is that unknown water heater or coolant sound I speculated earlier. Then, tons of rumbling from...nobody knows. This isn't some crazy phenomenon or spooky mystery. This is just the world of infrasound. Nobody seems to every know wtf they're hearing and are typically trained to recognize patterns of natural seismic events. Speaking of that, I'm excited to bring some of my kit to Yellowstone next month to, perhaps, if I'm lucky enough, be able to let my viewers be among the first humans to listen to enormous geothermal events and geysers!