Why is a light switch is buzzing at audible volume?
There are a couple of possibilities: the most likely is that the switch is worn and should be replaced.  If the switch makes a good connection sometimes and a bad connection at other times, you would hear the noise only occasionally.

If your local laws allow you to do the work yourself, I think this would be an easy task.  The steps would be (at least in the US where I am): 


turn off the circuit breaker;
remove the switch cover plate;
double-check that there's no voltage at the switch in both on and off positions;
unscrew the wires from the switch
screw the wires onto the new switch
reinstall the new switch and cover plate.


Another possibility, but much less likely, is that there's a loose connection between the switch and one of the wires.  The steps to fix this would be the same as for replacing the switch, but tighten the screws on the wires instead of removing the wires.