The Dumont is a split sound design? Do you understand the difference between split sound and intercarrier designs? There is a lot of literature. Here is and earlier Videokarma post
https://www.videokarma.org/archive/i.../t-251212.html
The ringing is most likely due to the RF local oscillator off. The split sound design you adjust the fine tuning control for best sound, not best picture and the best sound adjustment is fairly critical.
Try adjusting the local oscillator or fine tuning to bring in the best sound. Once you have the best sound, the picture should be optimum if after all these years the IF alignment is intact. Generally the alignment will be off especially if you have replaced parts or even replaced tubes or redressed the wiring in the Video IF stages.
But get the local osciallator on frequency first. If the RA103 has a tuning eye tube, that would be used for optimizing the fine tuning for best sound.