You can tweak up the power on a 24VDC switcher to cover the headroom for a
linear regulator following the switcher. This is an old technique - and
still used. My preference would be an adjustable regulator with the adjust
pin bypassed with 10uF. Linear Technologies has some regulator chips that
out of the box have noise lower than garden variety LM317/337.

You may have to shield the supply --