The hard disk makes the case shudder, the processor and power fan constantly emit a fuzzy roaring. This may not be a problem for a lot of users, but it can be a real problem for others. Here are some tricks I used to decrease the noise, sometimes even make it completely disappear. This is dangerous. Switch the computer of before touching parts inside and pull the power cable away.
Computers are sometimes very fragile.
Everything you do to a PC can destroy something inside it. Even just opening and closing it. Be prepared to have to pay for another sound card or another motherboard. When doing something to the ventilator of a processor, keep in mind some processors really need to be cooled down. Without a ventilator they can burn in a few minutes or at least stop working.