[gary rabbitt]

Coming to the party late. The noise I would bet 100% it is from a power line or something outside of the radio.  You will be chasing your tail around if you are trying to eliminate it from the radio. It's not being generated there you must go to the source.


The Philco Special Capacitor has a purpose as someone already said what it is for.  While you could use a plain cap and possibly not detect a difference, it was there to filter leftover IF that could leak into the B- and cause ringing, a howl or other weird behavior.
Lots online about the real reason for the Special capacitor.  And I have to say that cap/ filter is NOT to filter out the 500KC coastal stations that are now defunct.  THAT is a different wavetrap elsewhere in the RF section.

    Just a thought, take the Hippo to a friends house away from your neighborhood and give it a try.  I used to get the exact sounding buzz from a touch lamp when it was on. I don't know if they still have those things.