It's normal. It happens when it's running with no load (or almost no load).
More precisely, it happens when there is no continuous current through choke. If choke current drops to zero, it will ring at the choke's parallel resonant frequency, which is the inductance in parallel with the parasitic capacitance between the windings (plus some smaller parasitics).
Give it a load larger than half choke peak-peak ripple current, and all will be fine.