thanks for the replies guys.
I've had the same problem, and I've replaced the original Yate Loon fan with Scythe Kama Flex 135mm 1200 rpm; yes, the original fan is 135 mm, which heavily restrict your choices. One more observation: the original fan use a different connector than the Scythe fan, so some soldering will be required.
Some more details: I've estimate the original fan to run on full speed around 2800-3000 RPM; In my case(modded Lian-Li PC-A05N) the power supply has separate air intake & exhaust; I would recommend, if it's possible, to use the Scythe Kama Flex 135mm 1600 RPM, if the PSU has to exhaust directly from the case(no separate intake). Before the PSU mod, my other 7(5 Noctua NF-P12 and 2 on the Gigabyte GV-R585OC-1GD video card, all running around 8-900 RPM on idle) fans in the case were covered by the PSU fan noise, no matter of load; now the PSU fan noise is completely covered by the other fans.
does the Kama Flex run at 1600RPM minimum, and if so isn't that a bit fast (i have no idea, just wondering)? i was thinking of replacing it with a fan that runs at around 800 - 1000 RPM at low temps and then increases from there.
i'm not a gamer and use my PC mainly for work. while i'm working i use very little resources so system load is minimal and during this time i'd like as little noise as possible. when i render CPU use is at 100% for hours at a time, and during this time i expect the fan to ramp up and be a bit louder (though of course i'd still rather not hear it too much).
maybe i should just cut my losses and go for something like THIS