The 60Hz buzzing sound came from the impeller and magnetic drive assy,
of course. And the filter box acted as a resonator, amplifying the
sound.
I tried AquaClear and Aquaclear Jr. filters, and both made similar
noise.

I find the constant splashing sound of the water spilling back into the
tank
annoying, too. Maybe I should consider other options like a canister
filter.
Or I could get lucky and find an old Aquamaster on eBay or at a yard
sale...

"Excelsior, you fathead!"
-Chris-