loud, annoying beeping sound on 16th street

Hi Again,

I posted this a few weeks ago, and it got a lot of views but few responses. Nonetheless I am trying again, as last night the loud annoying beeping sound on 16th street was as loud and annoying as I've ever heard. It sounds like a little ping, a kind of sonar-like sound and repeats every one minute or so. Sometimes it seems more muted, last night it was quite clear and loud. I have heard from other folks that this is audible on 15th street as well. I have called 311 and I know it is not a smoke detector. Someone, somewhere must have an idea about this. Please help!