Something I've nostalgically missed from Tekkit Classic is how all of IC2's machines had cool sound effects when they were running, and I noticed that Big Reactors machines don't have any sounds at all. I found a few sounds that were like how I'd imagined reactors sounding:

https://www.freesound.org/people/digableplanet/sounds/131189/ or https://www.freesound.org/people/Timbre/sounds/169859/ for the nuclear reactor itself, and

https://www.freesound.org/people/CosmicD/sounds/33503/ or https://www.freesound.org/people/DJ%20Chronos/sounds/130291/ or https://www.freesound.org/people/jaredi/sounds/215307/ for the turbine.

I'd imagine the pitch of the sound being shifted relative to the machine's current operation speed (Minecraft has a built-in way to do this).

Please make sure there's an option to turn it off, preferably a volume control. I've found that sound doesn't work very reliably, and what seems like a nice sound in isolation becomes annoying when it cuts in and out.

Irl, reactors themselves have nothing to make noise with, other than coolant moving through tubes, boiling or rotating turbines.

Noted in the roadmap. It's something I've intended to do, but I'd rather work on core mechanics at the moment. Still, something to remember for the future.