Hi everyone, I've been using an Aquacomputer D5 USB since the end of last year, and so far so good, very pleased with its performance.
Except from a really strange behavior I've noticed in the last period, but I didn't have time to investigate. I normally run the pump all the time on a couple of fixed values like: 15%, 30%, etc.. depending on the task I'm performing, so I've quite never controlled it with a curve. Actually stays almost all of the time on 15%. Increasing the pump % speed in aquasuite, going up by +1% with the arrows, I noticed that when the pump goes from exactly from 20% to 25% values, makes a really strange and very audible sound.
I'll try to describe it cause it's not that easy to give the idea: it's like a granular, vibrating, high pitched sound that is for sure related to the rotation (impeller). In this 5% interval sounds completely different from every other value in the scale. It just performs perfectly at 60, 80, 100 % speeds, it makes a smooth zzzzzzzzz sound that just increases regularly going up. Example: if the value is set on 10% and I type 40% percent and press enter, I can hear very well the pump going through that interval. It starts with a zzzzzzzz, then vvvvvvrrrrrrrrr, and zzzzzzzzz again. The "vrrr" it's the strange vibrating sound it gets when it passes through the 20-25 % value, then the increment from 26 to the typed 40 sounds normal. It actually sounds louder at 22% than 60%, really. I personally wouldn't like to change with another pump, I'll be just skipping that functioning interval. Maybe leaving it running on those resonance values will settle down the behaviour, maybe. No trapped air, bubbles, no nothing, system performs well, flow values are OK.
I don't personally think (AND HOPE) the pump it's failing, but wanted to get some opinions from the community and maybe a "don't worry" from shoggy or AC staff. Has anyone dealt with something similar?

EDIT_PS: I fogot to mention that this wasn't happening at the beginning, 101% sure (too much percentage in this post, jeez). I did severe system, leak and pump testing after my last machine update

Hi Jason, that sounds very strange to me. My Aquacomputer D5 USB and aquabus, linked through aquabus, changes its power gradually from 0 to 100%, I can hear it, and especially measure it as I also have an AC High Flow sensor hooked up too.
Not reporting an incorrect value at all, going from 5% to 20% makes visible and audible changes. Also note that if i set it on "0%" it doesn't turn off, but rather runs at the minimum possible rpm, and I guess that's because it's powered by a molex which always gives a sort of minimum voltage/power to the D5.
As I told before I often run it about 15% when in normal/idle condition, as it's a great combination of performance and noise. High flow actually makes quite a very few benefit compared to other variables such as fan speed, especially in "long" loops with more rads and blocks. Going from 50 to 100% with a pump makes almost no difference in liquid temps° (maybe 1-2 degrees at max if you're lucky), unless at 50% you have a really awful low flow. Depends from case to case, but going 0.5 to 1 GPM makes a relatively difference most of the times, and the noise increases by a lot really, and that's not optimal for who cares about acoustics of its system, which for many is the main point and reason of watercooling.

Still I can't explain why my D5 started making this rattles and noises when running just 20 to 25 %, very odd indeed.