"Sweep 21" is a unique audio piece that starts with a resonating noise that resembles the sound of hitting metal. It's an abstract, yet intriguing blend of sounds that could easily be associated with a sci-fi environment. The audio morphs seamlessly, building suspense and anticipation. It then transitions into a wooshing sound, reminiscent of a spacecraft zooming past or perhaps an alien entity swiftly moving. This gives the audio a dark, eerie undertone. It's the kind of sound that could serve as an apt opening for a sci-fi show or a thriller movie. The sound, despite its abstract nature, is sure to pique the listener's curiosity and draw them into the narrative.