as other posters have said there is an inherent buzz in the trannie due to the line frequency of your supply. Usually it is not noticeable when new and all the winding coatings (varnish) are fresh and screws are tight. If it annoys you I have found a workaround by putting my trannie on some high density foam rubber to minimise the resonance of the wooden shelf that it sits on (like a sound board of a musical instrument). Or you could play loud music in the darkroom...... cheers!