The Flux Flow Oscillator (FFO) is a long Josephson junction in which a DC bias current and a DC magnetic field maintain a unidirectional viscous flow of magnetic flux quanta. The theoretical linewidth of the electromagnetic radiation generated at the end boundary is due to internal current fluctuations as given by the usual expression for the lumped junction. Experimentally, however, the linewidth deviates significantly both in magnitude and functional dependence. We suggest a simple solution based on the assumption that the bias current creates an additional magnetic field in the junction. This is supported by linewidth measurements on FFO's subjected to different bias configurations.