Tsunami or Loksound Cummins

Tsunami or Loksound Cummins diesel. No Conductor, though. You could put custom sound boards at the depots instead.

Rapido Loksound

I have a Loskound Select decoder with Rapido's own RDC sound file that I bought directly from Rapido a few years ago. Looks like they might have a LS5DCC version now:

https://rapidotrains.com/products/ho-scale/sound-decoders/esu-loksound-v50-decoder-budd-rdc 

Not sure if they lock out access to soundfile. If not, you could add your own sound recordings using the LS5 sound editor with Lokprogrammer hardware and software. 

Pete

Detroit Diesel

I think most of the RDCs were built with Detroit Diesel engines so they would sound a lot different than a Cummins. I imagine Rapido has that part down quite well so if you're striving for a realistic engine sound then probably best to go with them or create your own sound profile as suggested.

True enough on the sound. I

True enough on the sound. I was making an economic suggestion without shelling out $300+, especially since the OP already has the Proto model.  I'm not aware whether you can get the sound file from Rapido to put on your own decoder.

ESU Have a sound file for the RDC

Check out the following:

http://projects.esu.eu/projectoverviews/search?cat=18&q=rdc

it's for a Loksound 5- Detroit diesel.

Great! Thanks for pointing it

Great! Thanks for pointing it out.  Laziness on my part. I knew of the Cummins sound and never looked further.. Time to pull the old tsunami and put in a Loksound on my Proto RDC.