What is this (possibly electrical) noise?

Since last night at about 10:30, I’ve heard a very unusual sound three times (once again at around 4:30 am, and again about half an hour ago). It’s like a series of buzzes, three buzzes in rapid succession, and repeated three times – for a total of nine buzzes.

The buzz is low, sort of like an alarm, but the hour was inappropriate for an alarm, and it’s not on the half hour mark or anything. Also, it quits after the 9-buzz cycle each time.

It’s occurring in my bedroom, and for the first time, I have rather a lot of things plugged into the power bar in that room… though none of them particularly active. Could this sound be related to an overload of some kind? The apartment building is probably about a hundred years old. A Google search leads me to believe that such a noise would be constant. Does anyone know what else it could be?