Sine wave generator -
Judging by the frequency of the oscillations alone I expect it may just be parasitics of the system. Some parasitic capacitance is turning your transformer winding into an LC resonant circuit and it's ringing at a high frequency. I see this all the time and the best way to prevent it is to critically damp the system by adding resistance to the output. The value of the resistance may need to be determined by trial and error in order to get a favorable waveform.