Sounds like the end of a whip when it goes by.
Around WW 1 they put silencers on 1903 Springfields and set out to test the silencing ability. The range they were using had a row of power line poles parallel to the flight of the bullet. Though the gun blast was almost silent every time the bullet past a pole it would leave an audible crack from the faster than sound bullet. It went CRACK, CRACK, CRACK, CRACK, CRACK, Not to stealthy
I believe navy seals when using 9mm pistols use heavier weight, subsonic bullets so as to optimize the use of their silencers.