Question about Logitech Z533

You probably hear a so-called 50Hz hum which could be caused by potential differences in the earthing of different circuits. I had such a problem myself, which was only solved with the purchase of an expensive hi-fi subwoofer :-/
Try the following: Turn the plug of the subwoofer 180° (if it has no earth terminal). Try plugging it into a different socket in a different area of the apartment/house or even in a different house.

I also have this problem. When the subwoofer is switched off, it emits a whistling noise. As soon as I switch on the speakers, regardless of whether an audio cable is plugged in or not, the two speakers emit an annoying background noise. I have tried the whole thing at various sockets all over the flat without getting any other result. Does anyone have an idea for solving the problem?

The problem is the coils of the power supply and the low pass. Blast away the fabric cover (with a screwdriver under the edges and gently lever it up), unscrew the loudspeakers and then spray everything that looks like a spool with a generous amount of plastic spray (Kontakt Chemie Plastik 70 (Farblos)). Always helps. I completely eliminated the whistling in a few minutes.

Mine don't make any noise when switched off or on (without a signal, of course). Possibly change the cable routing...?

I have also noticed the same thing. I will probably have to replace the audio cable and/or build a noise reducer.

The same can be observed with the PlayStation 4. The simplest solution in both cases is to unplug the power cable when not in use. This also prevents possible silent power consumption.

Yes, the problem is also present here. But that's not a problem, almost every electrical appliance makes some kind of noise.

Depending on the LED bulb, you can even hear ceiling lights "howling".