I have the same issue and sent them this:
'I have just received a new AsiAir Pro from ------. Unfortunately it makes a significant buzzing noise when a dew heater is on power at any level other than 100%. I have two dew heaters which I assume are low current draw (being for an 80mm scope and 50mm guider). The issue occurs with either or both connected in any combination of power ports. It also occurs powering the AsiAir Pro from a mains supply or lithium battery. I previously had another AsiAir Pro which was completely silent powering the same heaters with the same power supplies. Could you tell me whether you think the unit is defective or is the noise a characteristic of recent production models (maybe you have changed the type or manufacturer of the power module)?'
The reply was:
'Sorry for the trouble, it is a known problem, and we will fix it asap.'
Working in software and dabbling in electronics I suspect this is a hardware problem. Whilst the noise is present using all ports it is louder on some than others which would suggest no soft fix to me.