5.5 kHz noise on SMPS output
Good question. People think that the only noise that can occur is the switching frequency and harmonics. This is not true. You are not the only one that has found noise below the switching frequency. Normal SM PS filter LC parts will work well at say 40 kHz but your 5.5 kHz will cut through it like butter. It will be better to nail this at the source than use a large expensive output filter. These limit cycle oscillations are sometimes associated with the resonant frequency of the output filter. Study this and study your control loop poles. Double check for interaction and change your loop or change your output filter. Loose ferrite cores can also cause audible oscillations despite running far above 20 kHz.