I won't lie, I have virtually no idea how hard this would be to implement coding-wise so at the moment I'm just spitballing some ideas. I think this would be a pretty big step immersion-wise, in my opinion its small details like these that really add to the overall quality. Plus, I think in the current state of the mod the trains are far too quiet, real trains create a clamor wherever they go.

Much in the same way that pack creators can swap or outright remove most default sound effects for each piece of stock (such as whistle, idle, chuff, bell, and wheel clack), these new sound effects would also be integrated into this modular system. I'm well aware that you can already add idle sound effects like dynamos and air compressors for steam locomotives but diesels lack this feature at the moment.
Even if the base mod's stock didn't include these new sounds themselves, as a pack creator I think just having the option to add my own sounds for these categories is a big deal.

New sound effect categories could include:

• Check valves occasionally releasing / air compressor engaging when locomotive throttle is at idle for a period of time
• Intermittent radio chatter that only plays when you're seated in that piece of stock (I could see this being used for more than just radios- maybe music or ambient passenger noise)
• Brakes squealing as they are engaged & hissing when disengaged (with an option to disable this sound in a rolling stock's .json if it doesnt have air brakes)
• Wheels creaking as stock starts to move from a standstill
• Extra track noise as stock moves over crossings/switches/junctions
• Intermittent squealing as stock moves over track curves (volume & frequency of this being played increases in conjunction with some sort of ratio of gauge versus sharpness of curve)

(these last two suggestions are probably the hardest to implement but I figured I'd just throw it out there in case its ever feasable)

Best,
Gibster