Looking for a smooth running great sounding HO DCC loco - early diesel era

I agree with Brakie.

For the pre 1960 era, I’d go with the Atlas ALCO S2, RS1, RS3, or their EMD GP7. They are equipped with Loksound, and except for a slight hum at low level sound switching speeds that you may not hear, Loksound are some of the best sound decoders for both sound and more importantly, IMO, operation.

Atlas made some H15-44’s and H16-44’s. Check eBay, you might find some great deals.