[Milsurplus] ??'s on BC-474

[Hue Miller]

Years back, i had to replace the audio output transformer
in the BC-474's receiver. ( I think the previous owner made
the mistake of grounding the receiver B- lead to common
ground. )  I used a transformer from a  Command Set receiver-
i think i actually looked up the impedance ratios and such,
to verify a reasonable match. It worked fine - however, there
was something like "fringe howl", altho this is not a regen
detector. I put a damping resistor across the transformer
primary to stop this audio tone.  My question: why did 
this howl arise in the first place? Is this also related somehow
to the radio's use of the battery tubes?
Hue Miller