So as you walk along Nevsky and the little alleyways around it, you will here "the noise". It's a metronome like sound that plays in the background just loud enough to be notice.
During the blockade of Leningrad, they would play this sound over the loudspeakers to let the inhabitants know that the city was still under control of the Russians. Didn't matter what time of day or night, this sound was broad-casted throughout the city.
Apparently, they still play it.