If your debating building a quiet computer this can still be done on a budget, just because a pc is virtually silent does not mean it costs much more to build.
There are a few points worth noting though, it’s not just a question of looking for parts which are actually designed to make very little noise, sometimes these can cost a premium just because of this.
Use fans with sleeve bearings as opposed to the ball bearing type, this will really help to make your pc silent. It’s worth noting though sleeve bearings do have a shorter service life than the latter, so they both have their good and not so good points.
How much do you really need certain fans? Many computer components can actually be cooled from just heatsinks without fans? , many manufacturers now provide this option.
Fan larger type fans, the rule of thumb behind this is, the larger the fan the slower it can go to create the same airflow as a smaller type.
Any extra cards such as PCI, make use of the amount of ports you have, leave spaces between cards, don’t bunch all the cards together you may think it looks tidier, but it certainly won’t help with heat dissipation and airflow.
When looking for hard drives, checkout it’s decibel reading, you can usually find this on the drive manufacturers website, also try to find a drive with a large cache, this is very useful for common functions, which cuts down on the time the drive has to read from the actual disk, plus it makes the drive appear faster.
Make sure the computer case has plenty of ventilation, Ideally it should have ventilation in more than one area, having too little ventilation obviously leads to excess heat, but also leads to increased noise levels as the air is sucked in and expelled, grills in a pc case can often be a major cause in making your silent pc not so silent.
Make sure any cables in the case are secure, otherwise they can suffer from vibration, and can even work themselves loose, larger cables such as IDE type can also obstruct airflow if not routed well.
Power supplies can cause a whole multitude of noise problems, they have really come down in price over the years and manufacturers have recognized that pc power supplies did have a noise problem in general. Today most power supplies are classed as quiet, but there are even some super and ultra quiet ones available.
One point really worth mentioning when building a quiet computer is HEAT if your system does not adequately dissipate heat, it’s not going to function as it should in terms of speed and it will certainly create more noise as any heat dissipating hardware such as fans have to work harder.