Perhaps RF is arriving via the wall wart lead.  Try running the wall
wart lead through a ferrite toroid several times as close as possible to
the receiver?

Is the frequency of the hum the same as power line frequency at your
location?  If so, perhaps AC line frequency is affecting the audio
ampifier; use a regulated DC supply with capacitors closer to the
receiver, or replace the capacitors in the audio amplifier.

--=20
James Cameron
http://quozl.netrek.org/
--=20
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