Just a theory ..

Looking at the WinradHD screen, there is clearly something what has not been there before: a nice clear signal at the 4.622.5kHz. It appears and fades periodically together with the UVB-76 carrier, i.e. it seems to be transmitted from the same geographical area.

This will give nice 2.5kHz constant sinewave if received in AM or LSB mode, which has annoyed the AM stream listeners for last couple of days. However, the voice messages were clearly transmitted in USB. So -- may be the buzzing has been replaced with a constant sinewave which is way easier to monitor with technical measures than buzzing.
This way, one could receive the LSB as control feed, measure the amplitude of the 2.5kHz tone and assume how well the transmitter is doing. USB can then be used for voice messages as needed... If I would be them, I would either use another tone on LSB for indicating when voice transmission is in progress ..

IN FACT!! As I type (2.50pm UTC), the signal at 4.622.5 started to modulate itself between two frequencys, 2.5kHz and slightly lower pitch, sounding like a teletype.

I hope I am not correct on this, as this would mean that we will not hear any buzzing on that frequency any more ...

Laid.