After the first time I thought that this is a device error, I sent it back and ordered a new one. Unfortunately, there seems to be a general problem, as the 2nd unit had the error also. Then I hear a lot of audiobooks, go in front of a lot of quiet places, then a very clear humming / buzzing noise can be heard, that bothers me a lot. The noise occurs only when Bluetooth connection on and disappears if you put the volume on from the speaker. Once the minimum level is selected is because the sound is not loud and also in increasing the volume. When connecting via cable, the noise is not there. Actually I think the unit from sound, functional, etc. not bad, but the noise is too distracting for me.
