If you had large heatsinks (can fit much larger ones in a bigger case) and a big low rpm fan that is designed with good quality quiet bearings and the fan itself is quiet then your pc would be really quiet.
Most people have incredibly shitty low quality cheap coolers in their pc (stock intel fans, low quality amd reference blowers, cheap sleeve bearing fans, small heatsinks)
If all you have in your pc are high quality fans that run at 300 rpm on idle and 500-700 rpm under load then you cannot hear it.
xbone could be quieter (and stay quieter), it all depends if they bother putting in a big heatsink and a high quality fan.
Which is highly unlikely for either sony or ms to do, they'll go with some cheap sleeve bearing fan that cuts the production cost by 50 cents so they can save some millions for their bottom line.
I remember how the xbox 360 usb cover was held on with a paper clip(that broke off the very first time I opened it), or how my ps3 fan got worn within 1-2000 hours of use.
You don't even get a half decent fan if you buy a 400 dollar gpu directly from nvidia or amd.
If you bought a 7970ghz reference model a year ago for 550 euros, it would make 55+ decibels of screaming noise.
If you installed a bigger heatsink with way bigger surface area and 1-2 high quality 120-90mm fans and set them to 500rpm with a fan profile then it would make less than 35db of noise (and remember decibels don't present a linear scale, 60db is exponentially much louder than 50, not 20 percent louder)
