Plinius 9200 - transformer buzz

I have a problem with my Plinius 9200. Buzz comming from it's transformer is occasionally very loud, annoying and distracting especially during low passages. It's not constant, for most of the time the transformer is silent. Have you experienced that with your Plinius? What could be the possibly reason and how to solve the problem. I've written to the manufacturer, to Plinius Audio but haven't received any answer.
Power supply 230V (Europe), dedicated PC (I have tried a couple of them on the amp).

Thanks