My computer's cooling fans make a terrible noise.
Please, could you give me some advice about how can I reduce that noise?
First, check if the CPU usage is low. For that, press Ctrl+Shift+Esc and open Performance tab. You'll see a chart. Wait for a few minutes without using your PC (for instance without playing videos).
If you see that the usage is high (25% or more), then something is using your CPU, which in turn leads to the fan noise. You'll then have to find what is using your machine and fixing it. IF you're unsure how to do it, SuperUser may be of a great help.
If the usage stays low (1% to 5% with occasional short peaks at 50% is OK), then the problem is usually with the fan itself.
In this case, you have to clean the fan with the help of a vacuum cleaner. Carefully extract the dust from the fan and the heatsink (the metal plate between the actual CPU and the fan). The fan may start to rotate very fast, which is normal.
Hairs, fur and large chunks of dust may be impossible to remove with a vacuum cleaner: if those remain, try to carefully extract the remaining dust with an ear Q-tip.
Proceed with caution:
Unplug the PC before opening the case.
Don't use anything which could conduct electricity (such as a screwdriver). Even unplugged, the PC uses battery power from a small battery on the motherboard, and a wrong contact could do a lot of damage.
Never touch the motherboard. There are a lot of fragile components on it, and damaging them is relatively easy. This applies as well to other parts such as the GPU and the RAM.
Try not to damage the blades of the fan: remember that fans are both precise and relatively fragile. While a slight contact between the vacuum cleaner and the blades shouldn't do anything harmful in general, there is still a risk to damage it.
Don't shake or push or pull the heatsink. Doing this can damage the motherboard, and eventually the contacts between the CPU and the motherboard if the PC is old.
Make sure you also clean the airflow holes of the PC case, which are usually found beneath the front panel, at the rear and on the side.
Organize your cables. None of the cables should be near the fan: if there is a contact with the fan, it will make noise, and could potentially damage the fan and/or the CPU (because of the overheat). Cables shouldn't prevent the airflow inside the case.
Generally you cannot do much with your current fans (unless they have some defect), I mean, they are generally noisy. But you can do a few changes:
purchase SSD - completely noiseless. Even if not directly concerning fans, it does help in reducing overall noise. It also produces much less heat than an ordinary HHD and reduced the need of fan work to some extent;
purchase special low-noise fans - some brands offer such and while not completely noiseless, the reduction of noise is tangible (speaking from first-hand experience).
Another possible measure is to simply place the computer case as far as possible so that noise simply does not reach you. This would come at the expense and incovenience of extra long-cables.
If the fans aren't defective, try some double-sided tape between the fan and the mount. I've used that with some Deltas and it reduced the noise dramatically.
This worked as Deltas are high-RPM, high-CFM fans. If there was any play between the fans and the mounts they would vibrate against the mount. If there wasn't any play, it would vibrate the box (in this case, an antminer). The tape dampened the vibration.