Noise comparison between the D5 and the APEX VPP
For anyone who hasn't heard of the APEX VPP:

The APEX VPP is not a clone of the D5 - it is an entirely different pump design and to my knowledge, not manufactured Xylem. It is a "refined" version of the infamous VP755.

Here is a video comparing the pump noise.

https://www.youtube.com/watch?v=ezTnY2Qlvzc

Details on the test setup:
Disto / pump / res combo:

https://www.ekwb.com/shop/ek-quantum-re ... -rgb-plexi

Additional comment: it has additional vibration dampening - I'm using longer screws paired with HDD anti-vibration pads to fully seperate the distro from the metal fan tray it is mounted on. The fan tray is additionally isolated with 0,5mm thick EPDM tape on all metal to metal connections and additionally, the screws having rubber washers.

Pump cover

https://www.ekwb.com/shop/ek-quantum-co ... -d5-nickel

EK says it is not compatible with the chassis / res - it is. You have to get creative with the dremmel on the plastic vanity cover for the PSU shroud, is all.

Installation:
Plug and play into the EK Distro. The cover also holds it as it is supposed to. Didn't have to tinker at all with the O-ring, fits snug, seals well, no leaks. No complaints here. Pump comes with an additional O-ring - in my case, I did not have to use it, as I reused the o-ring from the distro.

Differences on paper:
Minimum RPM for the APEX is 2170 RPM, PWM needs to bottom out at 0% in order to achieve that RPM. Depending on your model of D5, the minimum RPM might be different.

Max RPM on the apex is 4470 RPM. Max RPM for the D5 is 4800RPM.

The D5 has much better flowrate (1500L/h) compared to the APEX (360L/H)

The APEX (4,25m) has better head compared to the D5 (3,9m).

Build:
The APEX VPP has a plastic pump front (in comparison to the stainless steel [?] front of the D5) and there are seemingly no metal parts in sight. The axle seems to be ceramic. The impeller assembly is secured with a plastic c-clip onto the shaft. The back of the pump is some kind of metal, probably aluminium.

Pros:
The overall noise level is lower than the D5 throughout the entire RPM range;

The vibrations from the pump passed onto the chassis are much lower than the D5 (contributing to the lower noise level imo);

The noise profile itself is less annoying, it's more of a hum compared to the whine of the D5;

Cons:
The pump is not dead silent at low RPM, it's not too big of a deal IMO, quieter than a hard drive spinning at idle.

I noticed slightly higher temps on CPU, but no difference on GPU (5950X + 6800XT) - this makes no sense, but I feel the need to mention this.*

*This could be related with me tilting the case around like a maniac and dislodging the CPU block while switching pumps without draining the system (yes, I switched pumps without draining the system), the crappy thermal paste I used on the CPU block when I ran out of MX-5 or the BIOS update I did 2 days prior to the switch.

My 100% subjective conclusion:
Definitely not dead silent (one can dream...) when at 100% RPM, but leagues quieter than the D5 and a definite improvement, when the D5 whine annoys the hell out of you when it's running at full tilt.
https://www.reddit.com/r/watercooling/c ... _apex_vpp/