I tried to make a quick casual recording of rain and thunder outside the window today by setting my Zoom H4n on my desk (internal mics, 96 kHz, 24-bit stereo, lithium batteries). When I loaded the WAV file to edit it, I noticed a rhythmic noise below 50 Hz and -50 dB. I removed everything above 50 Hz, amplified by 20 db or so, and shifted the pitch up by several octaves so I could hear it, and the noise sounded just like a heartbeat. The spacing of the beats also corresponded to my heart rate at the time, which was about 67 bpm.
This surprised me. The recorder was resting on the desk. I was leaning with my elbows on the desk. So I reasoned that the recorder had actually picked up my heartbeat. This sensitivity surprised me, but it seemed at least plausible.
So I tried putting the recorder on a table across the room and started recording. The ambient noise level was around 38 dB(a). I walked in and out of the next room, doors open. Then I took a look at the recording. The "heartbeat" was there again.
Now that I found hard to believe. It's not a fancy digital recorder. The room was quiet, but not that quiet. I walked in and out of the room, but the amplitude of the "heartbeat" did not change. And yet it sounded like a heartbeat, and it matched my pulse.
So, what could this be? Is it really possible that a little recorder like this could record heart sounds even across a room? Or is this noise from some other source that just happens to sound like a heartbeat and just happens to match my pulse?
I don't have enough experience with sound engineering to draw conclusions. Can someone provide me with some insight on what this might actually be? The suspense is terrible!