I've been after some decent fans to bring down the noise level of my PC, but there aren't many decent roundup reviews I can find, with most of them being outdated. However, I recently stumbled upon these two reviews, which are excellent IMO.
Based largely on the above reviews, I went ahead and purchased a few fans to make a subjective comparison, a Noiseblocker NB-BlackSilentPRO PL1, Scythe Kama Flow2 900rpm, and a Yate Loon D12SL-12. I already have a couple more I'll cover, an Antec TriCool and a Sharkoon Silent eagle I wasn't completely happy with.
Bear in mind I'm looking for quietness here, but obviously airflow will play a part in the decision. Also I'm not using any noise/airflow measuring gear besides the back of my hand and my ear, but you could argue an ear is better than a noise meter as some tones are more annoying than others at the same loudness, however some noises may be more or less annoying/noticeable to other people. Also, when I mention problems such as vibration/noise, I'm comparing to other fans mentioned here - I don't consider any of them to be poor fans, just some are better than others, and YMMV - I'm only testing single samples. One other thing, when I mention ticking, I mean something like a faint but rapid tick/click for what sounds like several times per rotation - stick your ear next to nearly any relatively quiet PC fan and you'll probably know what I'm referring to.
It could just be my experience but I find ball bearing fans tend to be noisier than other types such as sleeve and FDB so I avoided them after the Silent Eagle. However, I remember a 92mm Akasa Amber as being very quiet, but it's a dual ball bearing fan which may be why. I may try some more at some point, the Scythe Gentle Typhoon being one which comes highly praised. There are a few more fans I'd like to try but most of the ones I have in mind are more mid-speed fans.
Edit: As CAT pointed out below, the Silent Eagle uses a sleeve bearing, not ball bearings as I originally thought.
Antec TriCool - When set to low (med/hi not really suitable for this comparison, they're both fairly loud) it performs fairly well; mechanical noise is nice and low, but airflow noise isn't all that smooth - you can hear a bit of air turbulence noise, probably because the blade design isn't optimised for silence. For a pre-installed fan it's pretty impressive and moves a fair amount of air. However, my one occasionally makes a sort of clicking/scraping noise which can sometimes be cured with a nudge, like parts are slightly out of tolerance or something. Also very little vibration, however I noticed a faint ticking noise only present when placed on a hard surface; applying force downwards didn't help (originally suspected wobbling) but it faded when lifted up or placed on silicone gasket. Edit: As noted in a post further down the thread, I've found the two TriCool 120mm fans I've tested behave badly when mounted horizontally, with airflow pointing upwards, but it doesn't happen when flipped. One is worse than the other, but both create far more vibration in this orientation to the point I don't consider them suitable for use mounted like that; aside from noise, reliability may also be a concern IMO. However, this is something I'll try to investigate further, as Antec ship loads of these fans mounted exactly like that in their cases. I should also mention I didn't notice the scraping noise in any of the 3 fans in my 900 case during use.
Sharkoon Silent Eagle - Again, for most people it will be plenty quiet enough, airflow is decent but seems to kick a percentage backwards. It makes a noticeable buzzing/rumbling at all speeds and some vibration so ideally should be soft-mounted, but it is supplied with rubber mounting pegs. I've also observed a scraping noise similar to the Antec; it's uncommon during use but I've noticed it a few times during spin-down.
Update: After using this fan for a week or so in my PC, a good portion of the rumbling noise has subsided, it still creates some vibration but it's on par with the Scythe now and should be fine with soft mounting. It rates better than when I tried it out of the box, but it still falls behind the Noiseblocker/Scythe; yeah it's 1000rpm vs 900rpm but even when I reduce voltage the noise doesn't change much, and if anything becomes more annoying as the softer noise subsides.
Yate Loon - At 1350rpm it's the fastest contender here (I assume the Antec on low is slower, it looks/sounds like it) so I'd expect it to be the noisiest. And it is, but it also moves the most air. As an aside, I'm not certain why they use the high speed model in some PSUs, I'd think this one would cope fine. Anyway, the most prominent sound would be the buzzing noise, but I don't think it's bearing noise as it ceases as soon as I cut the power, so likely electronic noise. And it's a real shame as undervolted to 6.5/7v it would be a good contender without it, moving a decent amount of air, similar to the Scythe. Also very low vibration, even up at 12v, so hard mounting should be fine. I may pick up another sample to check for that noise, as for the price they would be excellent for vertical mounting (sleeve bearings), but being more mass-produced (I assume) than the others, there may be more sample variance.
Scythe - Exceptionally quiet with a very smooth sound profile - I have good hearing and beyond about 15cm it's as good as silent. I can hear a faint whine during startup but it fades when up to speed, and can't really be heard at realistic distances. There's a small amount of vibration however, nothing significant, but you may want to soft mount as most of the noise generated came from vibration when placed on my desk; when lifted up or placed on the silicone mounting gasket supplied with the Noiseblocker, it's much better.
Noiseblocker - Joint quietest here but with a slightly different noise profile, a very faint ticking/buzzing noise is present but again, it really shouldn't be audible in practice. It would probably be the quietest here as it has very low airflow/mechanical noise but the faint tick makes it roughly as audible as the Scythe IMO as it's more noticeable than a smooth hum. Very little vibration so hard mounting should be fine, but you get a good vibration-reducing mounting kit supplied. Airflow is, according to my hand, slightly less than the Scythe.
Unless you're after extreme quiet and you have an Antec case, the TriCool fans on low perform admirably, unless you get a slightly poor sample like I have, as that scraping noise is quite audible and annoying, but I suspect it's not a common thing.
Unfortunately, as I said above, I can't recommend the Yate Loon ATM because of that buzzing noise but if another sample (I'll try to go with another supplier to try to get a different batch) is free of it, it rates highly in value for money if you're prepared to undervolt it, but if you have a fan controller it may be an advantage as you can turn them up for more airflow when needed. Also, aside from the Antec at mid/high its IMO more suitable than the others for a hotter syetem; super-quiet fans are more suited to Llano/i3/i5 systems without discrete graphics for 2 reasons - they may not expel heat too well, and the GPU fan will likely be much louder. Also worth noting is the use of a sleeve bearing; it's the subject of much debate in some places but they tend to perform well at first but wear faster than the others so end up louder than other types would be after a while. I'm not really sure of timeframe though and it would depend on the quality of the bearings - a very good quality sleeve would likely outperform a poor quality ball bearing even over time. It's also generally recommended sleeve bearings are only used vertically has they wear faster and can perform worse when mounted horizontally, but this doesn't seem to stop countless PSU MFRs from doing it...
And despite some good reviews, I can't really recommend the Silent Eagle either; it's by no means a bad fan, but even without the other three I bought to compare it with, I wasn't too keen on it. I'm probably more picky than most but there are better fans for the money. It's also scored well in some professional reviews but maybe that's due to what I was saying earlier - a higher number on a sound meter doesn't necessarily mean a more noticeable noise to human ears. I could have a bad sample but it doesn't seem like the sort of thing which would vary much. The vibration/noise seems more to do with design than anything being unbalanced or out of tolerance.
As for the remaining two, they're both excellent low-speed, low-noise fans and I'd have a hard time picking a winner here; for hard mounting I'd have to recommend the Noiseblocker, it barely vibrates at all and is incredibly quiet. For soft mounting, it would be the Scythe as I find the noise profile less noticeable and it has a slight advantage with airflow. However, we have a problem - the Noiseblocker is supplied with a great soft-mounting kit, the Scythe comes with normal screws. The Noiseblocker is the most expensive by about £3 but it's cancelled out by the need for some soft mounts for the Scythe IMO. For heatsink mounting I'd probably go with the Scythe, I think it would cope better with the resistance.
So I can't really call it conclusively, if you get a soft mounting kit, I'd go for the Scythe for case use, but for anywhere soft mounting isn't practical I'd recommend the Noiseblocker. But really it's very close either way, and I feel it would be unfair to call one a winner over the other.
Feel free to ask any questions.
Thanks for reading!