anyone know if a computer really would leak more fan noise if you cut a window into it, opposed to just having that metal sheet there? and what if you cut a fan hole as well?

I think it all depends on what type of fan you are using meaning what the decibels you would get from a fan and CFM, meaning the air flow the fan puts out the higher the air flow the louder the fan would become of course it also depends on brand and name fan you get.

There are so many fans you can buy that our ultra quiet ranging from 12db to 30db and higher.

The ones i have in my 2 x UFO-U2 cases which are Cooler Master running 22db with a 47.734 CFM air flow i have 10 of these in both of my computers meaning i have 20x120mm fans running when both pc are running not counting any other fans such as the CPU, Video Cards,Power Supply, Memory Coolers, Chip set Coolers and Hard drive coolers and i am very pleased with there performance and sound.

window, no.
fan hole, a little. it depends on the entire setup, but it probably would not be noticable to the human ear, except for the additional noise that fan makes.

oh, alright. thanks!

I'm with both Ranger and DN on this one...

There are so many deciding factors to take into consideration, one of which I believe DN touched on...
The entire setup, ie - The case's material/build quality and the hardware inside.
The fans you use - are they standard fans or super quiet? or performance ( high cfm )

Other things to take into concideration are how well the window is cut, placed and sealed.
Fan grill/No fan grill and your positioning from the actual system.

So many tiny things that can ammount to a large factor.

If it really bothers you, just line the side of the case which is closest to you with sound absorbing foam.