It sounds like the hummmmm in your ears

Recently someone in South Baltimore, maybe around Violetville, was talking about how she or a neighbor who are older use a device to keep young people believed to be involved in criminal activity or loitering away from private property. It emitted sounds at a frequency that can only be heard by younger folks with good hearing.

Perhaps it is something like that, aren't there still some old-times-were-good timers up there?

Mosquito

This perfectly describes a sound that I hear in a certain spot between my house and the grocery store. I'd be so pissed if I lived next door to someone that operated one of these all the time.

Heard it last night here in Fed Hill as well. I assumed it was a slow moving /idling train somewhere though.

Me too. I don't think they are very considerate of your neighbors. I mean, you're assuming a lot about your neighbors when you make a decision to use one of these. It wouldn't be kind where I live, and there is a mix of young and old.