The transformer will make some kind of noise, very slight, when the unit is on. Depends how loud your buzzing is because it shouldn't be noticeable unless you have your ear right up to the unit. Do you mean from the unit itself or through the speakers?
The buzzing is through the unit. It's only noticeable at absolute silence with the hear close to the unit. If it were a very cheap product i wouldn't care but for 300€ i guess it should be absolutely silent. Don't you agree?
I've just gone up to my C542 and I can only hear a very slight hum with my ear pressed to the casing. All transformers will hum to some extent, even the toroidal ones but the degree of hum can vary between identical units. This is due to tolerances in how the transformers are wound. If you're unhappy and think your unit is making more noise than it should and it's still under warranty, take it back and have it looked at mate.
I've followed your advice and tried another unit but the problem remains, what makes of it a feature instead of a problem :-).
The salesman told me to try a voltage regulator, but they are expensive. Does anybody knows manufacturers that make this equipments for HIFI or if the voltage regulators for computers (cheaper) are as good as the HIFI ones?
Can of worms mate. If you've got RF in your mains or any electrical concerns there are many on hifi forums who would advise against computer plug-ins and the like. Even most audio units will affect the sound of your system according to some. I use an RF shielding product and it works well, do some searching on the internet and make your own mind up.