SMPS, output ringing?
The inductor has got parasitic capacitance that forms a resonant circuit at a high frequency. The action of switching current into it and turning it off ALWAYS causes this to happen on pretty much ALL power supplies of this type. The leakage capacitance of the series MOSFET also interacts with the inductance to modify this too.

The inductor is 27uH and maybe the MOSFET has 200pF D-S capacitance - this would cause a resonant frequency of: -

f = \$\dfrac{1}{2\pi\sqrt{LC}}\$ = 2.16 MHz 

OK I'm just guessing at what \$C_{DS}\$ is but it's definitely in the same ballpark as you figures. You don't need to add a snubber - doing so will reduce the efficiency of your circuit and what are you trying to "protect" against?