I have never known RPM of a drive to affect its noise level. The fan(s) will be far more significant in noise level. Most drives run pretty quietly, though some can get noisy during I/O, like my HGST Ultrastar He6 drives.

Also, without knowing the model, I wouldn’t say they’re not made to run 24/7. But even on desktop drives, it’s rarely run time that kills them, it’s start-stop cycles. Everything will be fine, but one day you’ll shut it down and some drives won’t spin up. That’s why power outages can be deadly to an old server.