buck converter output

Where it was measured? Is it output with AC coupling? What is the time scale of the first waveform? The bottom waveform shows peak-to-peak >2V. That doesn't look related to the first one. Check your ground connection of the probe: when ground is loose you would see something like this. Also time scale of 5ns is not very useful: you need to check closer to switching frequency of the converter. Also check that you are not measuring noise in the ground fly lead of the probe: connect the alligator to ground of converter and touch it with probe tip. You may still see some noise just make sure it looks much smaller than what you are measuring. If it is the same - means you are measuring noise induced into ground loop