Yes, that’s normal.
This is the big transformer in a small box syndrome. All transformers make noise. The box they are housed in determines how noticeable that noise is to the user.
A big box made of wood will absorb a lot of this noise. A small steel box will vibrate in sympathy with the magnetic field generated by the transformer. The bigger the transformer, the more it will “excite” the steel enclosure.
The PS-100 has an unusually large transformer relative to the physical size of the unit. It therefore exerts a greater magnetic resonance onto the housing.
I hope that helps.
Dave