Talent shows and open mikes are fine promotional methods, but in the hiphop world, a buzz just as frequently begins with somebody’s boomin’ system. The first song I heard from local MC Interstellar (aka K-Oss) came blasting out of a friend’s speakers as we were headed down I-95 in his truck. I don’t know what the song is called, but from the back where I sat, head nodding furiously, it sounded like an instant underground classic. Though a car stereo is not the ultimate test of good hiphop, it is certainly part of the proving ground. Dubs of Interstellar’s as-yet-unidentified single are probably making the rounds in trucks, jeeps, and sports utility vehicles near you, but if you can’t get a ride, you can catch him live this Sunday at Nocturnal Promotions’ Hiphop Showcase. He’ll be performing with fellow D.C. rap acts Melodramatic, 5 Bro MCs, and Da Great Deity Dah at 8 p.m. at Metro Cafe, 1522 14th St. NW. $5. (202) 518-7900. (Neil Drumming)