Step Down Switching Regulator irregularity with ripple on output
It seems your load current is small enough that the buck converter enters discontinuous mode. This means in basic terms that the inductor charge is exhausted before the end of the cycle, hence you get a some ringing before the next cycle starts. 

Usually it's recommended you'd want to stay near the transition between continuous and discontinuous mode in normal operation. Theoretically this leads into optimal component sizes and costs but practically it's very much a moving target.

Wikipedia gives reasonably good explanation of what's going on there:
https://en.wikipedia.org/wiki/Buck_converter#Discontinuous_mode

To push the SMPS back to CCM, you'd want a larger inductance value. However changing the inductor size may also call for a smaller output capacitor. And then you need to re-adjust the feedback loop compensation as well.

If you're not comfortable with all that, I'd suggest


Leave it alone, DCM is not a problem as such, or  
Use Texas Instruments webench tool to design you a SMPS circuit, it does
practically everything for you. Kids these days.