I first noticed it off in the distance at 7:50 am (it is now 8:40) and going strong. It is around low G and is a steady pitch and tone. No changes in pitch or loudness. It is exactly the same each time. The duration is between 5 and 15 seconds. The tone is too dull to be any yard work or construction project. I've enjoyed listening to all the neighborhood sounds--the noise from machines doesn't bother me much. This one stumps me, though. What kind of yard or construction project would produce such a sound, where it is on/off/on/off/on/off...? Its regularity and unwavering quality is not like anything I've heard around here before. Wait, now I can hear someone gunning the sound a bit, before it ends with a half-second fade out and pitch glide down. Can't tell if it is electrically powered or gas. Definitely different from any construction or maintenance sound than I've heard before.