Hi,
That ringing is the diodes capacitance in series with the 33uH inductor.
This is very very typical for a buck, and i've seen it many times in the past.
If you add more capacitance (a little more) across the diode, you see the
frequency of the ringing go lower, and if you calculate the resonate frequency
lo and behold it's the same as the ringing
It happens because the diode stops conducting, and once it stops conducting
it's like an open circuit and all that is left is the capacitance in series with the
inductor. The mosfets capacitance may contribute a little too during that time.
It's not too bad as long as it doesnt go too high and thus exceed some components
voltage rating.
I almost forgot to comment on the inductor value itself...
33uH does sound quite low for 30kHz so it is good that the frequency was increased to
100kHz. The lower inductor value as well as possibly saturating too soon
also puts a strain on the output capacitor, and the
output capacitor needs to be able to handle the rms ripple current without any problem.
At 100kHz that situation gets better too.