Based on a real case effects of long-term exposure of infrasound on man are outlined. Beside a description of the background of the case together with remarks on the occurred health problems, the main view lies on the proceeding in identifying the special kind of exposure just as possible technical causes. As a source of annoyance a small heating plant was identified, which immitted into the house of the exposed people very low frequency airborne sound far below the common hearing thresholds. The results show clearly the general deficit of research on the effects of low level infrasound on man.