Yes they do hum, thay have an always on linear PSU and a soft switch type On button.
One I checked before placing this post, I can hear the transformer from ~100mm away.
@tautech: Thank you for the cross reference
Today I had a look inside.
The transformer ist screwed directly to the case. I put a 2mm foam rubber between transformer and case and now the noise is gone.
Beside this:
- no rust inside
- the smd soldering looks very good
- the internal layout/floorplan is well made
I don't know if such transformers are only tested with 110V/60hz and Germany with 230V/50Hz is a corner case.
Cheers
hammy