This morning (~1p) I was awoken by a very distinct, solid hum that appeared to be coming from nowhere. I sat up to see if I could place the sound and it got much quieter, almost disappearing. I figured the sound must have gone away, so I lay back down to go back to sleep and the hum returned. After a little bit of moving and curiosity, I managed to determine that the sound reached a maximum right where I lay my head and was otherwise mostly non-existent, which was very curious and made it very difficult to track the sound. This led me to believe that there was some sound nearby hitting a resonant frequency of the region where I sleep, so I went hunting for the hum, which seemed to be vaguely coming from upstairs. As soon as I got to my brother Joe's room, the source of the humming because quite obvious, his new rock tumbler. The rock tumbler's motor was operating at a frequency which, when placed above that particular corner of my room, caused a very loud resonance right where I try to sleep. Having figured out the source and having gotten about nine hours of sleep anyway, I don't really mind but it was pretty unpleasant waking up to a really loud hum and wondering if I was going crazy because it only existed when I tried to lie down.