Converter hum

It's probably just the fan, it will get louder the more lights (load) you turn on. Perfectly normal.

Mine was running about 48 hours straight.. knew they run a lot, especially in the evening with lights on, but this was extreme. Checked battery and it was hot to the touch, pulled the caps off, and it was smoking out from all 6 holes. End of battery. Shut off electric, got new battery. End of continuous running. Above suggestions all good, too. I was never told to clean or lube anything in the converter area. Learned the hard way.

Dynamo hum......

Might just be the cooling fan.... like beemerphile1 posted. Were your batteries low? Is this the first time you have run it in a long while...It might just be working hard to charge up the batteries and run other stuff also....

Give it a day or so to get "caught up."

Is it a cooling fan running?

Most electronic device "hum" is typically from a dirty/unlubed fan or from a transformer and it's separate plates.
Solutions: Clean fan blades and housing, lube fan with machine oil on bearing underneath shaft-located sticker (I typically just tape or glue the sticker back on).
If the transformer is mounted on four corner bolts, tightening the mounting bolts can reduce hum. Likewise, tightening any bolts that hold the transformer plates together and reinforcing with varnish may reduce/eliminate hum.
Beyond that, tightening all mounting bolts and, sometimes, adding a mass deadening mat can work.
Good luck with a fix.
BTW, ascertain that it is indeed the converter and not a buzzing breaker. That can lead a guy off course.