[MrFixr55]

To elaborate on Arran's comments: Try running 2 similar radios in the same location, in the same outlet. If it happens on both radios, then the issue is likely external, but could be power transformer arcing (that would likely transmit to the other radio, however, your radio does not have a power transformer). If only in the radio in question, it is internal.

Yes, the lack of power line filter caps can allow external interference in. early (late 1929s) AC radios did not have these. I could pick up the oven heating elements cycling on my Radiola 16.

If external, s the popping occasional or frequent? Do you have an electric stove or electric water heater?

SMD or any arcing in the radio can cause this. As Arran pointed out, a dirty tuning cap can also cause this. This most commonly presents itself as noise as the tuning knob is rotated, and is more likely to occur the more the plates mesh. If you turn the radio to the highest frequency (tuning cap completely open) and the noise goes away, the issue is in the cap. If the noise still exists, it is elsewhere.

The noise may be further isolated by removing the 2nd IF tube (on your set, you will need to bridge the filament connections with the appropriate 40 Ohm 5W resistor, or bridge and pray. If the noise disappears or is greatly reduced, the issue is not in the audio section.

Did you replace all the caps? Maybe replace the resistors. Swap the tubes. If no joy, you may be looking at SMD.