[davemc]

I had the same problems with a different set. It was due to oscillation in the IF amplifiers. In my case, it was a poor ground connection to a tube socket holding a tube shield. But in your case, it looks like the set uses loctals, so it would have no shields.

I would check for any bad tube socket or other riveted ground connections.

Another possible cause is LEAD DRESS. In some cases, it is critical. You might try using the set with under-chassis exposed, tune so you get a good whistle, and move around wiring and components with a insulated probe. If the whistle changes, that may provide a clue.

I always take a digital photo of the chassis before starting restoration, and attempt to put everything back where it was.

Good luck,
Dave