Nitin,
unfortunately the resolution of the schematics you attached is not very good so I am just commenting on what I can see.
First of all, there is nothing in the part itself that runs in the 400MHz range, anything at that frequency is due to parasitic, layout behavior and potential undamped components on the power distribution plane.
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I see you have an inductor at the output of the ADP2503 but no capacitance shown on the other side of the inductor.
That inductor could be resonating with the output cap and the capacitance after the inductor, that may need to be damped to lower it's Q.
Another source of noise could be layout related, for example if the input cap is not placed very close to the PVIN and PGND pins then any parasitic inductance could also cause high frequency noise.
If you share your layout we can take a look.
Do you have anything else on the board running at that frequency? Maybe a clock signal? that could excite the undamped inductor after the ouput also.
Best Regards,
LucaV