You could try a parallel resonant filter in series with the output. It does work. You have to tune the filter to the fundamental of the ripple. It works by presenting a very high impedance to the ripple. The only downside is that it only works at one narrow band of frequencies and if there are significant harmonics above that, they won't be affected.
The formula to calculate is (1/6.284) * (SQRT (C*L)), where C is expressed in Farads and L is expressed in Henries.
3.3μH in parallel with 4.7ΩF will resonate at 40.4KHz which is close to your quoted value. Note, the Inductor should have a very low DC resistance, preferably in the low milliohm region as this will affect the Q of the circuit.