[Heathkit] HW-16 audio troubles

[Tom NØJMY - AAR7FV]

I've got another HW-16 on the bench here that exhibits that hum, or 
hollow feedback noise, when the AF gain is not fully clockwise.  There 
was a thread on one of the lists about this awhile back, but I can't 
find it.  I remember one of the suspects was a ground loop somewhere, 
like a coaxial cable with the shield grounded at both ends.

Does anybody have any good suggestions on a trouble shooting procedure 
for this?

73,
Tom