[sdiy] what makes a transformer vibrate?

[Nils Pipenbrinck]

Is this a problem, and how to get rid of it?

I have a old Nordmende transistor radio from the 60th in my kitchen. 
Don't use it much, but yesterday evening I noticed how loud the little 
power transformer vibrates. It's the original external mains adapter, 
and I don't want to replace it. (it's just a 220V to 12V transformer, a 
fuse and 4 rectifier diodes, but the housing and inner life are still 
original and look very funky).

  Nils