Hi Diavolino
Servos are usually not very noisy, especially when they are "embedded" into the model.
The ones I use are the cheapest ones from China (cost like 2 to 3 Euros) and I drive them using ESU's SwitchPilotServo but the previous generation (they were much cheaper and doing extremely well. There is absolutely NO Noise once integrated.

One possible thing is improper adjustment: small servos don't deliver force and when a mechanical resistance exists (door already closed), then the servo tries forever to move it correctly and does a "humming" sound with sometime some hesitant moves.
I don't know the Faller servo controller but with the ESU you adjust 3 things:
- the open position
- the close position
- the speed of operation

Here is a small video during testing of my Car System servos to stop or divert the traffic
Here a bridge for Car System is upside down for testing
There is some background noise but the operation is very silent


Soldering of thin servo cables
The thinest I can think of is thin enameled wire. Even in this case it is easy to solder them to "normal wires" and of course use heath shrink (I have ordered quite a collection around and below 1 mm in diameter.

Another quick solution is to use Liquid electrical tape. It comes in a tin bottle with the cap fitted with a brush.
You simply "paint" the connection, let it dry and the result is a plastic/rubber coating to insulate. It is also great for marine applications (electrical connection on a boat)

Cheers
Jean