When indoor transformers are rigidly (not resiliently) mounted on the floor (or any other structure supporting them), their vibration will transmit to the support structure and finds its way in neighboring spaces and lower floor spaces (rooms, offices, laboratories, etc.),  causing an annoying, tiring, tonal noise as well as vibration. The transmitted vibration and its consequence, i.e., structure-borne noise can be addressed by:

• Isolating the core and coils (C&C) of the transformer from the floor/support structure, using proper vibration isolators. In air cooled dry transformers this means to isolate the C&Cs from the support structure. In an oil filled unit it means isolating the C&C from the tank base and isolating the tank base from the support structure.
• Making certain all the connections to the surrounding are flexible. This includes incoming cables, busbars, stand-off insulators, etc. Note that any rigid connection from the vibrating transformer to a solid structure will transmit vibration.
• Avoiding the use of a room, to house a transformer, with dimensions corresponding to half wavelength of the transformer vibration/noise frequencies. That is, stay clear of the room acoustic resonances being perturbed by the transformer.

Lowering the Transmission of Structure-borne Noise

Proper vibration isolation of a transformer from its support structure lowers the transmission of its vibration to the structure and thus abates the structure-borne noise. Vibration isolation of a transformer involves the following steps:

Study the flexibility of the support structure/floor and see how it would affect the vibration isolation effectiveness of the isolators.
Note that even the most rigid of structures are only rigid up to their first resonant frequency.

• Selection of vibration isolation scheme (single stage isolation or two stage isolation). This decision is normally made depending on the spectrum of the structure-borne noise, rigidity of the support structure, etc.
• Design of the brackets to allow the structure and cabinet of the transformer can accommodate the isolators, and
• Design of the stops in case the resiliently mounted transformer moves more than it should (in the unlikely event of an earthquake, for example).

Lowering the Transmission of Air-borne Noise

In addition to addressing the structure-borne noise, one needs to abate the transmission of the air-borne noise. This is mainly done via the acoustic treatment of all surfaces (walls, ceiling, and floor) of the room housing the transformer by sound barriers and sound absorbing material. Moreover, soundproof doors should be used for that room.

Structure-borne Transformer Noise Reduction by Air Isolation(PDF) Vibration Isolation | Sound and Vibration Control