It began with the slight yet forceful reverberations of a gong. For five minutes, percussionist extraordinaire Thor Harris elicited the low hum of his instrument while no one else occupied the stage. Then drummer Phil Puelo came out to add another percussive layer. The lights dimmed and brightened to match the rhythm, almost like a pulsing heart. One by one more band members of Swans entered the fray. The song, “Frankie M,” like all of their work on performed Monday night at Union Transfer, lasted in excess of 20 minutes. To call it or any of the other pieces simply a song would be to dismiss their nature; they were mini-rock orchestrations, hinting at the essence of the music itself and the boundaries of the human ear’s comfort level with their powerful sonics.