Andy has a good trouble shooting checklist, but I recommend that you save yourself a few minutes and disconnect all the circuts as the first step. I'm betting it's the Univolt itself. If it isn't, use Andy's method of resolve the problem.
However, even if you do resolve it to a circut, the Univolt will still be buzzing. Replace it per Lumatic's suggestion. The new unit, what ever you buy, will wiegh less than 1/4 of the Univolt and your batteries will last more than twice as long. It's a good investment.
Old Univolts aren't totally useless. If it's still putting out 14 Volts (its voltage regulation is poor without a battery hooked to it) or so, keep it in the shop to supply 12V
for testing other components, like pumps, etc.