Just Want Silent 120mm Fans

These are just about the best quality (and silent) fans on the market. They are not the prettiest fans, but they are well worth paying a little extra for.

Sorry about that, moving a little too fast
Updated.

There should be an extra cable that will plug into a USB header, or other motherboard plug, that will regulate the fan speed. But even at stock speeds, it is REMARKABLY quiet.

http://www.frozencpu.com/search.html?mv_profile=keyword_search&mv_session_id=965FZrvy&searchspec=SCYTHE+120MM&go.x=16&go.y=17

I bought my Scythe Fans from this very reliable site.

Arctic Cooling F12 PWM fans. Really quiet even at full tilt while still moving a lot of air: http://www.newegg.com/Product/Product.aspx?Item=N82E16835186033

with the only issue being that the PWM Circuitry can cause and extra noise - this is easily bypassed though.

A lot of companies "lie" about their measured fan specifications. One reason that Yate Loon fans were so popular was because their performance was actually really close to what was advertised; Corsair does a pretty good approximation as well. The reason I say "lie" is because, much like Mean Time Between Failures (MTBF), how a company can measure that isn't held to a standardization. Two fans rated at 30 Db could very well operate at 30 Db, however, one would operate at 30 Db at 1m, while the other might be at 30cm.

Also, two fans running at 30 Db can definitely sound different. I recently acquired at Corsair SP120 fan, and have been testing it. I'm surprised at how pleasant it sounds, because even though it still is loud, it is in such a way that it is noninvasive, if that makes any sense.

All this said, I'm sure both of these cases are the issue with your stock HAF XB fans: they're probably not actually running at 21 Db, and they probably emit a weird tone that you just can't groove with, even if they still are considered "silent."

As for the Cougar fans' extra noise from the PWM circuitry, it sounds like a higher-pitched whir type of noise. It definitely dominates over the sound of the fan itself. The way you bypass it is by not using the PWM signal; so either plug it into a 3-pin fan header, or use some sort of software (or hardware) that can control fans specifically by voltages alone. Motherboards that have the built-in "Smart Fan" utility I know can do this. Speedfan, a downloadable third-party application, can also do this. Honestly, the bad PWM circuits is a major bummer, and this issue shouldn't even exist... but I don't get any appreciable difference in performance between my Cougar fan and my Corsair SP120 High Performance. Since they are also robust, and can be frequently had for $12-15 dollars, I don't mind recommending them - I am just always sure to include the caveat.

Arctic fans are cheaper still, although it shows in their design, but still perform great. That's why they're at the top of my list. (Although I haven't had the privilege of trying a Noctua fan, I can buy *at least* two Arctic fans for the same cost. )

EDIT:
Food for thought - https://www.youtube.com/watch?v=c_qqZy2tNpA
https://www.youtube.com/watch?v=Vpsv4goSVQk
https://www.youtube.com/watch?v=aPY2IGVJoY8