I installed Linux on it and ran it. And it made this annoying periodic humming noise. It's not loud, but it is annoying because it is not constant - it is periodic. About 2-3 seconds on, 1-2 seconds off, then 2-3 seconds on again, repeat. This was very annoying. I can deal with it if it's a constant noise at the same sound level, but because this noise appears and disappears at regular intervals, it is extremely annoying.
So I decided to replace the stock HP CPU fan with a custom 80mm fan (I measured the stock HP fan and it is a standard size 80mm fan, no tricks there).
I replaced the stock HP fan with the TL-B8W ThermalRight fan. The installation is actually very easy - the TL-B8W comes with very long screws which go in the fan holes, and they do not screw all the way into the HP motherboard, it is actually very hard or impossible to screw them all the way in, I only used two screws, because using all 4 means I can't fit the HP CPU shroud back on (because the screws don't go all the way in). Because the screws don't go all the way in, this means the fan can rattle around if you shake it. Anyway, after switching to the TL-B8W fans, initially the noise went away, but then after a while it came back. I then went into the HP BIOS and set the fan speed minimum speed to 10%. This seems to have basically solved the problem - the periodic noise is so quiet that it is basically not noticeable with this setting, though the noise is still not completely constant - it still randomly fluctuates a little bit, so it is not completely silent. I then changed it to 20%, and the periodic noise returned. At 30%, the periodic noise was still there, but there was also a quite loud background noise added (I'm guessing that's the CPU fan running at 30%).
I also tried the Noctua NF-A8 PWM 80mm fan. This one can be annoying to install because you need a screwdriver whose shaft is narrow enough to fit through the fan's screw holes. With this fan I used the stock HP screws - they work. I didn't try the Noctua screws because they looked like they wouldn't work. Anyway, if you have a narrow shaft screwdriver then installing it is extremely easy. Initially, I decided to remove the rubber pads on the corners of the Noctua fans to make them easier to install. BIG MISTAKE! It turns out those rubber pads are actually EXTREMELY CRITICAL for noise reduction. Without the rubber pads, the fans were actually quite noisy. Anyway, I learned my lesson and put all 8 rubber pads back on the fan. I set the fan speed minimum idle to 0%, and I noticed the periodic noise. I then set the fan speed minimum idle to 10%, and the periodic noise was gone but replaced by much quieter regular pulsating noise that occasionally became annoying, though on average, most of the time it was less annoying than the periodic noise, but if I turn my ear in the direction of the fan, it becomes very noticeable. I then changed it to 20%, and the periodic noise returned, now much more noticeable than before. I didn't care to test more after that.
Anyway, it is hard to announce a "winner" because neither are particularly good, though I will say both are probably better than the stock HP fan. I think the TL-B8W at the 10% idle setting is probably the least annoying / bothersome noise pattern (easiest to ignore) since it seems to be more random (the repeating period seems to vary rather than being constant) than the other noise patterns, so I think it is the "best" although still not particularly satisfactory. I think it will be perfectly fine if you put it in a separate room, but if you sit in the same room as it, then you will definitely notice the annoying noise if you point your ear directly at it, but if you sit so that your ears are not directly facing the back of the case, then you might be able to forget about the noise sometimes.
The noise of this computer is WAY MORE ANNOYING than the constant quiet background noise produced by the HP 800 G6 Mini. Absolutely it is way more annoying. I can sit in a room with the G6 Mini running all day and the noise is so quiet and constant I would not even notice it, I would even forget that it was running. With the G9 SFF, the noise is WAY MORE ANNOYING AND NOTICEABLE.
I would not recommend the G9 SFF to anyone. If you do decide to get it, definitely don't buy it new like I did. I am dumb.
