We study the nature of extremely narrow-band signals that appear in typical long-time spectra of seismic recordings. At the German Experimental Seismic System (GERESS) array, we observe several of these time-continuous spectral lines. The most prominent one has a frequency near 2.083 Hz. The close agreement of this frequency with one of the feasible rotation frequencies of synchronous machines (50 Hz /24) suggests an industrial origin of the signal. An observed signal gap of 11 h duration is thus interpreted as a temporary machine shutdown. Interestingly, the signal vanishes (or is at least strongly reduced in amplitude) in the whole eastern part of the German Regional Seismic Network (GRSN) suggesting (1) that a single source dominates the signal in that region and (2) that the narrow-band signal propagates to distances of more than 300 km. On the other hand, we study the character of the wave by performing a suitably adapted array analysis. Estimates of the propagation direction suggest a source location near the German-Czech border. We obtain an estimate of the apparent velocity near 4 km/sec. Comparison with transient arrivals from regional earthquakes suggests that the narrow-band energy propagates as regional phases of Lg and Sg type.