I think the converter is going into discontinuous mode of operation at no load. With any reasonable load it's in continuous mode. Continuous mode is when the frequency of operation is fixed or at least stable. At no load though the output current gets so low that the converter has to skip pulses in order to maintain control over the output voltage. It's normal for many converters and not harmful. The only thing you could do is keep a little load on it.
My experience is that most time transformer buzzing is caused my the metal EI laminations vibrating. They try to minimize this by taping and gluing the laminations. But it usually occurs with heavier loads.
