I love how cheeky a title like “Knock Knock” is, coming from the anonymous artist behind one of last year’s best beat tracks “Sicko Cell.” Who’s there? To recap: “Sicko Cell” (or “I’m The Information,” or “Cocaine Powder,” depending on who you ask or what you feel like calling it) is the uncredited anonymous track — a great one, by the way — that both James Blake and Oneman spun in radio DJ sets, immediately setting dubstep forums and comment threads into attempts of frenzied futility at identify its producer. Identity aside, this next Burial or Zomby figure (assuming it is not Burial, or Zomby, or someone you know like Joy O or whomever) cut a track in “Sicko Cell” that is an essential drop in DJ sets that can accommodate such greasy, post-dubstep/post-whatever beat stuffs. Today FACT mag points to its b-side, by the same, anonymous (for now) producer, on a single set to be released on Swamp81. The track ties to “Sicko Cell” in its eerie airy synth swells, gorgeous pacing, “sicko” beats. “Knock Knock,” doesn’t matter who’s there, this stuff is magnificent. Hear both sides of the single below, if you like your music with beats.