There are three micro-switches on the latch board. When the door is shut, two of them should be closed and one should be open. If the third one fails and remains closed, it creates an unintended circuit loop that causes hum, although not necessarily operation at full power. I replaced all three switches (since the unit had to be opened up anyway) and the problem appears to have been solved. It may be intermittent however, as it did occur once in the past and went away on its own. The current assumption is that it the switch was still the culprit and replacing it should make a lasting repair.