Question about Teenage Engineering Op-1

When the OP-1 is switched on and then you hear the loudspeaker very clearly, do you also hear a very low frequency? Alternatively, play only one note and then listen to the loudspeaker. The loudspeaker should not make a sound, but I hear a very low frequency. When I connect the OP-1 to headphones or a stereo system, there is no hum at all. Thank you for a short feedback.