On 24/02/2014 16:45, DerbyBorn wrote:
I used to have a separate doorbell transformer. During some kitchen work
the electrician fitted a new transformer inside the consumer unit. I can
now hear a (50Hz)buzz in the bedroom above.


Probably the laminated core of the transformer rattling.

A drop of varnish on the laminated core often stops the
rattling/buzzing. Even nail varnish or a bit of blu-tack may do the same
job.


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