[heero_yuy]

The switchers we design for industrial panel equipment can sometimes make a noise at high line input and low load. It's a normal consequence of the supply trying to square the input / output circle and the pressure to cut costs. The operating frequency can fall low enough to be audible.

Just on the odd occasion we've had clients mention it and only once raised it as an issue. For that client we had the transformer specially impregnated with additional varnish to cut down the volume to an acceptable level.