[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.