There's been at least one lengthy discussion on this, just in case you hadn't searched yet.

I use Panaflo fans in my case, 2 in the back, and one in the power supply, with a variable speed YS-Tech fan on the heatsink. I have a Thermalright AX-7 heatsink. Alltogether this cost under $100 online. I can only hear the computer if I'm sitting at it, from 10 feet or so away you can't tell it's on.

It's likely your HSF is the culprit, they usually are because of the extra work that they need to do. I recommend getting the best heatsink you can, and then a quiet fan to put on it. Same results as an OE heatsink with a super-mega-loud fan.