If you look at the capacitor inside the IF can, the primary and the
secondary has very little insulation or separation between the primary to
secondary capacitor plates. There is a great DC potential from the plate
connected primary to the grid connected secondary. The potential is higher
in your Hallicrafters than the usual AA5. That is where the silver
migration or contamination occurs (from primary to secondary). Some times
scraping the migrated silver or tarnish off will solve the problem. But I
bet it would be only temporary.

I have fixed plenty of these "Thunder storm in a can" problems.

http://www.radiomuseum.org/forum/if_...er_repair.html
http://www.ppinyot.com/if_transformers.htm

And as others have said, discrete component failure (non IF cans) can cause
this symptom as well.

Good Luck,
Paul P.