[Tucker]

This radio was restored by somebody else about 8years ago and has played well up to now. I’m now getting a “squeel” or howl on the stations. It’s worse when the radio is cold and just minimal after the set has warmed up.  I’ve tried a different 6sa7 and 6sf7 tubes. The howling was less after replacing the 6sf7 tube but not completely gone. The noise also goes away when I hold a loop antenna up next to the radios loop antenna. Am I missing something simple?  
I think my next step is pulling the chassis and check all the resistors and test all the tubes and visually inspect everything else.

[Radioroslyn]

A couple of things come to mind. Check bypass caps around the mixer and IF amp also lead dress. If that seems OK then realign rf and IF stages and you may find that detuning the IF slightly will make it disappear.

[Arran]

I agree it sounds like one of the bypass capacitors has become open circuit, most likely a solder joint failed. Either that or someone "restored" the set using surplus parts, something like NOS paper or oil caps.
Regards
Arran