There is still the shaft turning, and ventilation, plus some other mechanical operations to maintain depth so it’s not quite that quiet.
Nonetheless, it’s all buried, oftentimes, under a thick rubber skin or some other soundproofing.
As you suggest, much more difficult to detect via noise.
Oh yeah, believe me, while I’m not a submariner, I have enough technical background to reasonably well understand the various systems that would be running. “Listen to your battery” was very tongue-in-cheek, but worthwhile in making people think of how quiet a boat on batteries might be, vs. one with a running nuclear plant.