An SBD that can be heard quietly only if there is no noise around. Sometimes, as during testing situations (a la SAT, GRE, ACT, school exams, etc.) a fissah will quietly but steadily be heard, the sound similar to a small broken air valve - "fissssssssss."
Last October while I was taking the SAT, I was straining very hard to concentrate on this reading passage about the artistic ramifications of celery when I heard a fissah. After that, my score must've dropped at least 70 points.