So what makes a pc loud apart from the hdd, CPU fan and normal circulator fans. What makes the pc noisy also is it worth getting a water cooling system?
First, your motherboard should support quiet or silent thingy.
Your fan should also be slim and light.
Your HDD should also be slim.
And of course, the CD/DVD drive should be exact, like don't go and buy ASUS ones because they're noisy when they read discs.
Use large quiet case fans and a quiet Power Supply
The stock fans that come with retail CPU's are also noisier than the branded ones (e.g thermaltake and zalman fans )
If your PC is noisy, you can replace some of the fans with your own, starting with the CPU and case fans.
If it's still noisy you might want to consider a water block for your GPU or a passive cooler if you don't play games.