buzzing power supply

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Hi!
the power supply of my poweramp makes a buzzing sound when I turn it on (guess it's about 50 hz, the mains frequencie here in (most of) Europe). It lasts for only a few seconds, and after that it's only audible when I push my ear against the amp's case. This buzzing isn't really disturbing me, but I'm just curious what might cause it, and if it is harmful for the amp in any way.
keyser
 
It's probably the transformer laminations that are buzzing.
Since it lasts for only a short period when power is applied, I'm guessing that the tranny is delivering lots of current while it's charging the electrolytic power supply capacitors. Probably nothing to worry about.
 
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