100Hz noise from a mains transformer
The base frequency of auditable noise of a 50 Hz transformer should be 100 Hz since the two half waves (negativ and positive) are giving the same movement in physical space on the moving part of the transformer.

Mechanical fixation and/or sound insulation is the way to go. Apart from screwing the transformer down, use some liquid glue (e.g. potting compound, protective lacquer) to cover coils and iron sheets. Be careful to use non-flamant, temperature resitive materials for potting. Also try to insulate the sound propagation to the housing by mounting the transformer on a foam pad.