Mysteries: That low mechnaical hum from behind Sixth Street

An EV Grieve reader writes in with the following...

I live on 6th between Ave A and B, north side of the street, middle of the block. Our bedroom in the the back of the building. The past few nights I've noticed a low mechanical hum that rises and falls like a wave. It starts at about 11 pm and continues through the night (or at least the next few hours I was awake).

It is almost definitely coming from outside/backyard. Or possibly the basement? I've only been in the apt. for three months and have only noticed it in the last three days, if that gives any clues. My girlfriend hears it too so that rules out delusions...

Ever notice this or hear other complaints?

I have not. In such situations, I usually blame ConEd. Or maybe UFOs/fireballs, like the one spotted last summer.

Anyway, do you have any ideas or explanation as to what this may be?

Report: Fireball in the sky above the East Village

From an EV Grieve reader last night:

Two people just saw a fireball in the sky moving from west to east above the EV. Was sighted from the rooftop on 3rd street and ave B. Both people said it looked like a burning ball of fire, but did not go out as expected as it climbed from 50' to several hundred in seconds.
Thought it was a Chinese lantern at first, but if that was the case it should have blown out. Couldn't be a flare because it was a flame and not dropping sparks.

Anyone see this? Do you have any ideas or explanation as to what this may have been? (And no Lady GaGa jokes, please.)