Wait till you assemble one using components from the market.
Prebuilts are usually built quiet as they are meant to be used in productivity areas anyway, and assume the user will be using airconditioning.
Generally, they are not as powerful or as versatile, so they can get away with lesser cooling. Both DEll and HP also use custom cooling solutions. If you order a PC from Dell with custom parts, it ends up at the same level of noise as an assembled PC.
And even then, I find them noisy. It is not necessary for background noise to overpower music, it just needs to be there to raise the noise floor, this affects reproduction quality. Try keeping your volume turned low so you can hear the music, without straining - a comfortable level. Now, turn on the ceiling fan. The clarity of the sound takes a nosedive. The same thing is experienced (albeit at much lower levels) during late-night listening when the PC is humming along.