Smoothing out a switching DC power supply
A typical switcher will be operating well over the 20KHz that we can hear.  (Humans in general, not me.)  Unless it's skipping cycles, it's probably not your problem.  If it is, the easiest and most reliable way to eliminate switching noise is to follow the switcher with an LDO to drop it another volt or so.
I'd look more at the AC connection and inputs to see if you're picking up the hum there.  Try shorting the inputs right at the connector--if the hum goes away, you have to start tracking it back from there.  If not, it's getting in somewhere else--at that point we have to know a lot of detail about the circuit and environment to help you.