Hi,
As awt mentioned there could be a problem with the transformer. Some transformers do vibrate and make noise too so sometimes rubber mounts are used. This usually happens in higher power circuits though, and sometimes sound deadening material is used inside the enclosure to help reduce the ambient noise which can be very very annoying to a human operator.
One of the causes is a DC offset in the transformer primary, but it could be in the secondary too due to the output circuit drawing an asymmetrical current. To find out if this might be the cause, you can try loading the transformer with just a regular power resistor and see if the noise goes away. You would need to load it to the same value as expected in normal operation.