Bachmann S2 Sound Value is noisy and buzzing.

[faraway]

The buzzing sound when the soundfiles are muted has been fixed. On a suggestion by Johnny aka Brokenrail over at the Bachmann forum....switching CV 217 to 0 solved the problem....CV217 on the Soundtraxx SoundValue decoders for Bachmann controls BEMF...setting it to 0 turns it off.
So now its just a matter of tweaking the running gear to quieten the gear noise.

That means the buzzing sound happens only when the motor is running. The is no buzzing sound if the motor is (totally) stopped. Right?
(I understood at first the motor buzzes even if stopped, that would be a serious decoder malfunction)

That is not a problem to be concerned but more or less normal. Especially at low speed is the decoder hard working to get the motor under control. That creates more buzzing sound as more mechanical tension/resistance has to be overcome. Switching BEMF off inhibits that process but you are loosing a nice part of DCC decoder functionality.
You should switch it on again after you have completed the inspection and optimizing of the mechanical components. A smooth running running makes it very easy for the decoder and the decoder will not need to control the motor with harsh.
CV 212 controls how harsh the decoder will control the motor. A lower value makes the handling more gentle and less noisy.