The older ones and many of the cheap ones do hum (both inverters and converters). The newer ones use better "parts" that are more energy effecent. If it has suddenly started humming then it may be dying. We had one on our pop-up that would keep me awake at night (pitch was just right), so we removed it (but then we also rewired the entire camper too). If the noise bugs you then shop around to replace it. There are several sites on the internet where you can learn about converters (or inverter, if that is what you have). For electrical (AC and DC) I like www.amplepower.com
as it gives a lot of good useful info. You might also check out www.phrannie.org
for the "Poop Sheets" for tons of RV info.