The older black and orange impeller DDCs are also a bit more buzzy than the latest blue or blue/orange types. The D5 has a more silent motor controller, but I've found they can be made pretty darn quiet if dialed down enough.
I haven't a clue how they could be louder than 2000RPM fans though, different tone for sure, but louder?
This is what I got in my pump noise testing round 1 when fully decoupled from the case:
The 35X could get way down there if dialed completely down and good at lower speeds. A DDC3.2 could also get there when a thick acetal top like the Koolance was added.
Notice the older DDC1 (black impeller) is a bit higher in noise compared to the current generation blue impellers (35x and DDC3.2), it has an older motor controller that is clearly higher in noise.
D5 with thick acetal top was pretty darn quiet though and quietest at full speed.
One thing to notice is sample variance, I tested two D5s (PMP450), the sample 2 had quite a bit of harmonics issues at certain speeds. Also notice the harmonic spikes in the 450S, this is due to sample variance and impeller imbalance, some samples are just better than others.