Typical flyback conductive noise emission simulaton
The MOSFET has capacitance between drain and source when it switches off. It forms a resonant circuit with the inductor and you get a sustained but decaying sine wave produced. This happens on all fly-back circuits.

So, if your inductor is 1 mH and your capacitor is 70 pF you will get it ringing at 600 kHz. Your MOSFET (STP8NM60) is specified as having an output capacitance of 100 pF so, not a million miles off.