They will buzz when hooked to a rheostat or a proportional thermostat (when the proportional thermostat is applying less than 100% power). I had the same problem when I had my ultratherms hooked up to my metal Freedom Breeder rack. Changing the t-stat to a grounded one solved the problem.
Heating elements have a high resistance factor. Applying a rheostat or proportional t-stat changes the resistance.
Try this: un-plug the UTH from the rheostat and plug it directly into the wall. Is it still buzzing?
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