It could be measurement. you should use a high frequency probe and measure directly across a output ceramic cap and see. The key is to make the measurement loop as small as possible. if it is still there, there are a few things you can do to help for you case:
1. Put more bypass caps close to you load IC
2. Put some RC snubber between SW node to GND. Usually C=~100pf to 1nF, R=~100Ohm to start with. But you need to be aware that the power loss is going to be higher so you need to check the switcher temperature to make sure it is in the range