Questions about a design for an SMPS post regulator filter circuit?
The approach is sound, with a problem: you have created a very "stiff" noise source capable of both driving and sinking whatever current is necessary to sustain its voltage.  If you think about how SMPS circuits work, you will probably agree that they themselves are charging internal elements and discharging them into the load rather than presenting a low-output-impedance voltage at two distinct levels.  If you don't want to simulate the output stage of your SMPS, I would suggest that you try a triangle wave for your noise source as a better analog for the SMPS output, without R1 in your circuit but with a series resistor for your inductor's resistance.