Well..... It is humming because it done forgot the words....
Ok, serious answers.
Converters humm for three reasons, there are usually two of them. But I'll list all 3.
1: Most converters have a fan, Some motors, if the bearings seize up or the fan is otherwise prevented from turning the motor may humm. That said... It's rare for a fan's motor to be noticable. So this is most likely not your problem
2: Load/overload. Some converters will noticablly humm at high load. If your batteries are dead or a cell is shorted This is combined with 3:
3: Loose transformer laminations, This is what is actually humming. The transformer core is made up of many iron plates, they are coated with adhesive and pressed together Some Transformers are made with better adhesive, some are not, and as the adhesive (Varnish is common) ages it may fail and allow the plates to move about a very small amount, The result you hear as HUM. By the same token metal near the transformer can be affected by the magnetic field and hum. Shock mounts are often used to reduce this hum by abosrbing it and they can wear out as well.
Note that this becomes worse under heavy load (See 2:)
Unless the hum is caused by an overload (IE: shorted cell in battery) the solution is usually replacement of the converter.
IF, and I stress IF, you have flooded wet cells (The kind you add water to from time to time) I would like to suggest a Progressive Dynamics Intella-Charger with charge wizard. I've researched a lot of them and it appears to be the best. Visit the PD web page and read up on it yourself if you like. If you have any other kind of battery however avoid this package. I'm not sure how the Intella-charger W/O charge wizard is but the charge wizard forces an equalization from time to time and this is harmful to all but flooded wet cells... Harms them too for that matter, but since you can add water the benefit is greater than the harm