The hum is due to a flaw in the audio output circuit design. Plug in
an external speaker and it will go away. There's a resistor (R134 or
139 - can't read the lousy schematic, but it's connected to J7) you can
bypass (short out), and the hum in the internal speaker will be gone
completely with an unregulated wallwart. No one has been able to figure
out what the original Lowe designer was attempting to do with this
circuit by connecting the internal speaker capacitively to the B+ rail
instead of ground (which is what the external speaker connects to when
plugged in.

Frank
K3YAZ