Best way to filter DC-DC output noise
Generally speaking, high-frequency noise will go straight through a linear regulator. There's a temptation to think of them slicing off everything above a certain voltage but they don't operate like that.

Inductors can be used to block the propagation of noise waveforms but the resultant load-side rail needs to be very well decoupled. This is because a transient demand for current by the load will be blocked instantaneously by the inductor (to view it simply) so the capacitance must supply this transient current in the very short term. I put a good amount of bulk and high-frequency capacitors across the load rail to effect this.

I can't say this is the definitive solution but this approach has worked very well for me on many boards.

I had to produce quiet analogue supply rails from a DC-DC converter output. I did so by putting a pi filter (C-L-C) between my +15/-15 V DC-DC and +12/-12 linear regulator and then another pi filter on the output of the linears leading to the analogue signal conditioning load circuitry and a generous number of 10 uF and 100 nF capacitors distributed well across the board, close to each load.

In my pi filters, the C was an assortment of parallel capacitor values (1 nF, 100 nF, 47 uF). But this also ensured enough capacitor sites on the board for me to experiment with other values during operational testing and EMC testing. The L was 10 uH. I also had 100 MHz bead ferrites between the DC-DC output and the first pi filter. (I prefer not to paste/redraw the circuit for confidentiality reasons but this describes it well enough.)

Again, I can't say this is the definitive solution but this worked very well for me on this board.