However I fear that the main reason for the buzzing is that the
transformer (of US origin) was designed for 60 Hz ,whereas here in the
UK (like in all European countries ) the supply frequency is 50 Hz.
Can someone on this NG please confirm/reject my suspicion.

I suspect that you are right.

If I recall correctly, transformers designed for 60 Hz can suffer from
excessive quiescent current flow in the primary windings, if used on
50 Hz... the windings don't have enough inductance. Also, because the
frequency is lower, the transformer will be drawing more current
during the brief intervals (100 times per second) when the power
supply rectifiers are conducting and recharging the filter
capacitors... the abrupt changes in the current flow and the resulting
magnetic-field changes may be causing the windings or laminations to
move around and buzz.

Transformers designed for 50 Hz service generally work OK on 60 Hz...
it an "easier" job for them.

Transformers can also buzz if there's anything on the same mains
circuit which is generating harmonic currents (e.g. other linear power
supplies). Toroidal tranformers can be prone to buzz or hum if other
loads on the mains line are drawing current from the line in an
asymmetrical fashion and thus creating a DC offset on the mains... the
toroid goes into saturation.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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