From the moment you turn on the 360, the fan makes a loud noise and never lets up until you turn it off. It quickly renders the idea of using the 360 to play music or watch videos faintly ridiculous, unless you want every scene that doesn't involve a marching band or an exploding helicopter to be accompanied by its wailing.
The PS3 is quieter than the 360, but in the hush of the lounge, it certainly isn't silent, especially when it's upscaling DVDs, playing DivX video or games. The tone of its noise is much lower and easier to live with than the 360's hoarse humming.
Noise levels will differ from PC to PC, but now 120mm fans are commonplace in even the cheapest cases, and quiet and inexpensive HSFs are easy to find. Reference coolers on newer graphics cards such as the 8800 GT are also better behaved than on earlier cards, so there's no reason for your computer to sound like a Chinook. That said, the number of fans involved in any PC - in the PSU, cooling the CPU and GPU - means that true silence is very difficult and expensive to achieve, so we can't give the PC a perfect score here. In contrast, the Wii barely makes a sound, and the fact that it's designed specifically for playing games means that even when it does make a noise, you'll be too busy waving your hands around to notice.
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