Since this radio has several 24A tubes try switching them around. The detector stage may have cathode to filament leakage. This shows up more as tubes heat. Others may not have this leakage?
Norm
:I have a 1931 US Radio Corp., model 26P and after it's on for half an hour it buzzes real loud just like a car radio would when getting interference from high voltage lines. I have replaced the filter caps and tubes already. The radio plays fine for about 1/2 hr then starts buzzing.