[EE] Metcal noise?

For those of you who use Metcal soldering stations...

I have a used MX-500. It works well (heats up quick, correct temp,  
etc), but does a couple of things I'm curious about:

1. When plugged in and NOT powered on, it makes a faint buzzing/hum  
noise...maybe 60Hz.
2. When powered on and heating the iron, the buzzing/hum is louder.
3. When powered on and heating, the back of the unit gets warm.

Normal behavior? Sign of impending doom?

Thanks.

-Pete
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