just noticed that a light switch (mechanical, not a dimmer; 220v 50hz) is buzzing at audible volume (can hear in 20cm), when both keys are on (first: traditional lightbulb, second: 23w energy saving bulb - with all that electronics inside).
Didn't opened the switch so dunno how it is wired (common phase.. or a common ground - if an installation point is powered by two wires, a drunk specialist may have used them the wrong way), it is known that this connection is split from wall sockets/main room lightning (yes, two switches outside the house!).
- pressing key 2: energy-saving bulb enabled, no sound
- 2, then 1 (normal bulb): sound.
- 2, ..some time, 1: sound
- 1, 2: sound
- sometimes, no sound at any combination.
Guessing that a bulb is possibly broken and sends a high-freq noise that forms an arc.. Unfortunatelly can't get a single normal bulb today to test if it forms a noise, too.