[Boatanchors] Transformer Question

[Keith Densmore]

Hi Group,
A question about transformer hum.
I have a HP 23 supply which has a slight hum coming from the power 
transformer.
Not objectionable if mounted on the floor but I have it installed in a 
custom metal enclosure and I do not like the extra noise.

Is there a cure for this? Or is a slight hum a normal aging effect on these 
older units?

Thanks, 73,
Keith, ve3ts