The loudest annoying device inside computers that is consantly spinning is usually the hard drive. It has a whine that annoys the ear drum more than even the power supply fan or CPU fan. If you have a few samsugn spinpoint quiet hard drives or similar this really helps.
Hold a fan in your hand and run the fan. Notice how the fan is quiet.
This is isolation.
Don't spend money on quiet fans until you verify what is causing the noise. Isolation of the fan may solve more of your noise problems. A quiet hard drive might solve more of your problems. Spend money on the quiet fans only after you've tried isolation and done hard drive bearing research.
Of course, the power supplies that are sold as 'quiet' may use isolation as part of their tactic - so that is a good buy if you can't isolate your power supply fan yourself. But the quiet fans that you buy to install on a heat sink, or a PSU fan that you purchase separately from the PSU itself which claims to be "quiet", probably won't affect your noise levels much unless they've done something on that fan such as install rubber isolation barrier between the mounting points. If it claims to be a quiet fan and it looks just like the fan you currently have with no rubber or signs of isolation, beware - i.e. fans that claim to have quiet "bearings" aren't going to help as much as isolating your fan.
Isolate your fans in your PC. It requires rubber mounts and rubber strips. Either custom made, or ones purchased for the job.
Use a thin strip of rubber in between the hard plastic fan and the heat sink. Don't cover the whole heat sink in rubber, just make a thin rim strip that stops the plastic fan housing from conducting noise to the metal heat sink.
The power supply unit (PSU) fan can be taken out of the PSU and mounted outside your computer with rubber spacers. This will require a bit of work and some thinking. If this is too much work for you at the moment, see what you can do inside the PSU first such as the rubber strip/rubber mounting trick.
Warning: careful fiddling around inside the PSU when you take it apart. They are not meant to be taken apart and have a warning on them.. because there are capacitors and other high voltage storage components in there. Wear rubber gloves and only plastic handle screw driver - only carefully take the fan out don't stick your fingers in anywhere.
The hard drive will probably be your biggest problem since some hard drive spin at really irritating noise frequencies. Try once again using rubber stripping to mount the hard drive.
Be warned that rubber stripping stops noise, but also stops heat conduction. This is only something to think about with the hard drive. Having rubber on the fan doesn't affect air flow! You aren't worried about heat conduction with the fan housing - it doesn't have the purpose of conducting heat. But a hard drive will run a bit cooler when mounted to your metal casing directly, since metal to metal allows your case to act like a heat sink for your drive. I've run my hard drive inside a plastic box encasing and it has not failed in 3 years, so putting a bit of rubber on it will not harm the drive - but I'm just warning you that rubber does stop heat conduction.
Also take note that the heat sink gets hot at times, so get some rubber hopefully that is higher temperature rated, or carefully monitor whether your heat sink is softening the rubber you put on. If it softens, try other rubber. Softening of the rubber once hot or warm, is a sign that the rubber can't hold as high heat, obviously.
How to find the noisiest fan in your PC and how to figure out what to spend your time on:
- stop the fan while the computer is running. You can stop a fan by sticking your finger in slowly to the center of the fan where no blades are and pressing on the flat piece of the fan. This is not recommended if you are not experienced with doing this. Your finger could get hurt.
- once you stop a fan, mark down which one makes the most improvement after stopping. Set your priorities on this part of the PC.
- put your ear up to your hard drive and compare it to your fans by putting your ear up to the fans.
One reason some laptops are very quiet is because the hard drive is enclosed and blocked off by tightly fitted plastic barriers. On desktops, the tolerances are looser and the case is not sealed off as tightly as a laptop. If you open up your laptop and run it with some covers off near the hard drive, it will be much much noisier. The hard drive on a desktop, once again, is probably going to be the hardest thing to quiet down. Since you can't modify the hard drive itself, and if rubber doesn't quiet down the hard drive for you, you can get a plastic box and enclose the hard drive. You will have to make a hole in the box to allow the wires through and you may have to get creative.
If you make a hole in the hard drive enclosure box you make, remember you can always unsolder some wires or uncrimp them and remove plastic connectors before feeding the wires through, then resolder/recrimp the wires once they are inside the box. You can also get a little plastic surround that fits in the hole to make the wires feed in more snuggly. Radioshack or electronic component stores sell plastic wire router pieces (seals/inserts) that are designed to route wire through cases.. they pop in and snap in place - there's probably one on your power supply as an example of what I'm talking about - it is snapped into a hole drilled in the powersupply casing, and lets the wires out of the power supply. Don't pull it off, just note what it looks like and go get some smaller versions of those for your hole in the plastic casing for your drive).
Putting the drive in a box will make it hotter though. Try instead to find a quiet drive that uses special bearings. Samsung spinpoints were one of the quiet brands.