If it's a converter/chrarger, then it is probably just keeping your batteries charged. This is why it's a good idea to check your electrolyte level monthly.
Yup, check you electrolyte level in your batteries. Mine started turning on the fan when I'd turn on the lights last August while plugged in. I thought, "How odd". Well I went out to check on it a couple weeks later in the garage and had cooked the battery. 1/3rd of the electrolyte was gone, OUCH. Thankfully my battery dealer had 125 Ahr AGM's "Blems" for 136.50 out the door.
"Confidence is the feeling you have before
you fully understand the situation"
1999 Toyota Tacoma Pre-Runner
TorkLift Tie Downs
2010 Travel Lite 690FD
2007 Honda ARX1200T3D
2008 Yamaha Zuma 50 Scooter
2010 "Blue" Bulldog, "Luna" Camped 10 Days 2012
The power converter does, in fact, run all the time when it is plugged in. It is supposed to.
There are, for the purpose of this post, basically two types of converters, Modern 3-stage converters are USUALLY switching power supplies, I won't go into how these work but they do not usually hum at you.
Some older units have a big iron core transformer, These may be multi-stage, or not, The core is made of layer after layer of iron "Leaves" which are varnished. The varnish holds them together, but with age and heat and vibration (induced by the magnetic field of the coils) the varnish fails and the leaves vibrate.. and hum at you.
It does NOT indicate a problem.
Nothin adds excitment like something that is none of your business
Kenwood TS-2000 housed in a 2005 Damon Intruder 377