Unwanted oscillations and distortions in buck converter
It could be that the inductor is resonating not with the filter capacitor but with the parasitic capacitances in the transistor and the diode, it could also be that the ringing is being fed back into the LM393 through R10, you mentioned that the ringing was on the base as well? (the output resistance of those things isn't that great). What happens if you use a second 393 as a buffer between U3 and Q1?