Hissing sound on a smps in my experience is either poor primary - secondary coupling within the transformer, or poor transient response causing oscillations within the feedback circuit, these however with your circuit are unlikely as your using a pre-made transformer, and the uc3843 is a current mode device making feedback oscillations less likely.
I suspect something is going on with the transformer, do you know the saturation current?, do you know the airgap?, the topology of the circuit the donor transformer came from could well be totally different than your application.
I see you have ringing suppression on both the pri and sec windings, how did you work these values out?