Univolts hum because they don't know the words. Install a songbook in the Univolt compartment...
Now, for the "non-smart" answer. The 60 cycle hum is caused by the insulation around the plates inside the Univolt deteriorating. Short-term, this will have no effect on the operation of the Univolt, but the state of the art has advanced dramatically over the last decade or so. it would be a really good idea to replace it with a modern converter, preferably with 3 stage charging. The new ones weigh about a tenth that of your old Univolt, so you will immediately add to your cargo carrying capacity, and the life of your coach battery. The old Univolts tend to fry the insides of batteries...