It's dead silent. And I do mean that. At idle, the fans are @ 500rpm, at loads only reach @ 800rpm.
Its the single biggest misconception there is about liquid cooling. Most assume that fans need to run full speed, but with a radiator having exponentially more surface area than any aircooler, the capacity for cooling and the efficiency of cooling is also far greater. So while actual temps may vary according to the effective pairing of rad and fans, the greater capacity means temp curves are somewhat different.
At 200w, the NH-D15 is approaching 80% capacity, at which point temp curves go sharply upwards to 250w. A 280mm AIO at 200w is closer to 60% capacity. Still has plenty of room. Much shallower curve, lower temps in comparison.
Meaning the fans are going to spin far slower. And quieter, especially if they are decent fans to begin with.
Undeniable fact. You cannot over-cool a cpu. Excess cooling ability results is ability to lower fan speeds, and therefor noise. An i3 with a NH-D15 means the fans would barely be moving, no matter what load was applied.
My rads are designed for low rpm fans. Thir best efficiency is @ 700-800rpm. They get better temps at higher rpm, but the efficiency drops off with every rpm over @ 800, so temps don't drop as quickly. Some rads are designed for higher rpm fans, they do lousy at low rpm but drop temps faster with 1800rpm or more. There's much more to rad design and fan pairing with a loop vs an AIO leads ppl to believe. Often times Noctua isn't the best fan for the job, even if they are the best fans.