Thanks.. I guess I forgot that noise can be from anywhere and could be introduced by the neighbor using a blender a few feet (or inches) away or something like that -- obviously depending on a number of factors..

 

The magic sometimes is adding common-mode chokes on both input and output power and ground since conducted EMI can be ground noise currents caused by capacitive coupling from the switcher switching transistors to the grounded heat sink.

But radiated EMI is the worst since it's picked up by every conductor in the assembly.