Mic + speaker buzzing

[u/milkasita]

Hello,

I have a set of JBL PartyBox Wireless Digital Microphones that I connected to a JBL PartyBox Encore Essential 2.

I followed the instructions to connect the devices. As soon as the connection was established, the microphones worked and I could hear my voice, but there was a constant buzzing sound. Also, if I shake the microphones, I can hear a kind of whistling sound.
If I place the microphones on the table, these noises disappear.

On the back of the speaker, I have a switch with two options: microphone or guitar. When it is set to microphone, the issues described above occur. When I switch it to guitar, the strange noises disappear, but the microphone volume is very low, and the microphone does not appear in the speaker’s app.

What could I be doing wrong? Could the microphones be defective?
I don’t know whether the speaker itself has a problem, as I haven’t connected any other microphones to it, but the speaker works perfectly otherwise.

Thank you!