You're talking about a CONverter, not an inverter. Commonly referred to as a "Univolt" on trailers the vintage of ours.
It's pretty common for Univolts to hum, because of the way they work. It'll probably keep getting louder until it eventually just stops humming (and at the same time stops working) but that may not be anytime soon.
The worst thing about the old Univolt is that it's hard on batteries if you leave the trailer plugged in for days. It doesn't back off when the battery is fully charged, so it'll boil the electrolyte, shortening the life of the battery.
A modern 3- or 4-stage 45-amp converter-charger is around $140-$170 dollars. The one I recently installed is a PowerMax brand, purchased through BestConverter
. I'm happy with the results. People also seem to be universally happy with the performance of Iota and Progressive Dynamics converter-chargers.