You need to put low-impedance loads on the outputs in order to minimize the effect of DC (resistive) and AC (capacitive) leakage through the 4066.

CMOS 4066 Question
It can be observed that large power transformers hum more under load. For example, when some devices are powered up, there is brief "twang" sound: a loud hum that quickly decays as the filter capacitors charge. Increased continuous hum from a transformer can indicate that the filter capacitors are leaking.

This question isn't about why transformers hum, but why do they hum less, or hardly at all, when not loaded. How is the electro-mechanics playing itself out to reduce the hum? After all, the fluctuating magnetic fields are there whether or not there is a load on the secondary winding.
