I have sent an email to the technical support (of the DC-DC converter manufacturer), he reply as follow:
the noise coming out of the input would be common mode and a single inductor may actually make the noise more difficult to manage; he suggest using input bulk caps with low ESR at the switching frequency and ceramic caps... and if necessary a common mode choke,not a single inductor.
I was wondering why common-mode noise issue arise with respect to a non-isolated dc-dc buck converter? usually common-mode noise occur in isolated dc-dc converter, right?