Josephson currents have been observed to drain instabilities and decrease the sensitivity of a heterodyne superconductor-insulator-superconductor (SIS) receiver operated at submillimeter wavelengths. A static magnetic field is usually applied on the SIS junction to reduce the magnitude of its critical Josephson currents and enable quasiparticle mixing. We measured the SIS mixer output power at the intermediate frequency (IF) versus the dc bias voltage of an array of two 1&mgr;m2Nb/Al–Al2O3/Nbjunctions. We show evidence of additional, magnetic-field-sensitive noise, peaked at the Shapiro steps voltages, which is due to high-frequency Josephson currents converted to the IF by quasiparticle mixing. ©1997 American Institute of Physics.