Hi Aussie Allan and Ben,
I have researched on this phenomena of noise. Here's what I found. It is generally, but it certainly helps for understanding the situation. Finally, I hope ....
The major noise sources are transformers and capacitors.
Transformers can produce audible noise, since they contain many physically movable elements, such as coils, isolation
tapes and bobbins.
The current in the coils produces electromagnetic fields which generate repulsive and/or attractive forces between the coils.
This can produce a mechanical vibration in the coils, ferrite cores or isolation tapes.
The Ceramic capacitors can also produce audible noise,
since they have piezoelectric characteristics.
If there is still too much audible noise in burst operation after gluing or varnishing the transformer properly, the capacitor in the snubber network should be examined.
By replacing the ceramic capacitor with a film capacitor,
the audible noise can be reduced.
In general, human ears are most sensitive to frequencies of 2kHz~4kHz and are less sensitive to frequencies which are higher or lower
The fundamental frequency of burst switching can be changed by modifying the feedback network, with a circuit modification.
Enough to argue in front of a salesman or even better, front of a company technician.
Gloup_Gloup
