Why does the Bluetooth adapter generate hum?

Hello, my problem is as follows: I have paired my tablet with a Bluetooth adapter (no name and not AVAIO) to transfer music from the tablet to the music system. The adapter is connected to a USB power supply. The output of the adapter is a 3.5 mm jack to RCA on the spare input on the amplifier. The connection between the adapter and tablet works. When a music track is played on the tablet, it is reproduced perfectly via the amplifier and speakers. However, as soon as the music track is played or stopped, there is a hum (roughly estimated at 100 - 200 Hz) in the loudspeakers (only after approx. 10 seconds - when the adapter goes to stand-by).
How does this hum occur and how can it be eliminated?
I found a user on Google who had the same problem. He solved the problem by using a power bank instead of the USB power adapter. This did not work for me. Neither with nor without the power-bank power supply.
Many thanks in advance for your help.Best wishes from the Westerwald and a happy new year!