And another half-dozen times since noting that. Low-frequency, bursts of a second or two, like when I was living on 10th avenue and they were excavating the new water main some distance away. Directional (turning the head affects it), very low-volume, goes a fair distance into the room and through shutters without diminution. Likely either construction noises or aliens come to wreak vengeance on the species after some imagined insult in the SETI signals. On the assumption that it's the former, planning on enjoying a good dinner over a philosophy podcast.
