[LAU] Buzzing noise from speakers when no audio app is loaded

Hello, although it is not a linux audio related issue, maybe some of you guys have experience with it anyway :) I have a laptop, connected external speakers which use an external power supply. Furthermore I have an external monitor, which is connected to the power plug. Furthermore I have a desk lamp and a Roland SH-201. All devices are connected to the same power bar. Nothing fancy, it just has a switch to convieniently switch off everything at once. As audio system I use Pipewire. As long as no audio app is using the "Build-in Audio Analog Stereo" sink where the external speakers are connected to, there is a buzzing noise. I tried to figure out from where that noise originates, by pulling plugs, rearrange plugs, pulled plugs of the TV and the internet cable modem in the other room (which is anyway not linked to the same circuit/fuse), but the noise does not go away. In some situations it got even louder. As soon as I start an app that connects to the audio sink, like Ardour, a browser with a sound playing, a decend "pop" can be heard and the buzzing noise is gone. Audio arrives at the speakers just fine, no buzzing noise. Once the audio app disconnects, a few seconds later the buzzing noise is back. Some years ago I almost used the same equipment, same room, only the SH-201 was added, and the laptop connected to a docking station. That time that noise did not happen. What might be the issue? Any hint appreciated. KR, Chris