A Funny Thing On Franklin Avenue

Today I was passing through the Franklin Avenue subway station for the A, C and Shuttle trains in Brooklyn. I had time to take my time, so I stopped in the main station area, where I noticed a very strange sound. 

It was so high-pitched that I imagine most people might not be able to hear it. It was so high-pitched that as it pulsed on and off, it sounded like a sharp snap as it penetrated my ears, jarring enough to generate the beginnings of a slight headache. 

But it was not so uncomfortable that I wanted to find what was causing this sharp swishing, seering sound. So I walked in its direction. At first I saw the large white speaker or fan or whatever, above.

But I quickly realized that the source of this regular, piercing sound was the smaller dark green box to the left, above. It's some type of speaker. 

What on earth is it? This is a piercing sound, in the most literal sense: it pulses on and off with about a two second duration, then a one second delay, then on again. The sound is almost outside human ability to hear, but it must be disturbing for dogs and cats nearby, as well as for young children. I'm not sure why I could hear it, but I could, or even perhaps it is true to say that I winced as I felt it.

Later: I'm wondering if it could be some kind of rodent and roach deterrent?