You could look at the Lian-Li PT-Si01, 02, 06, 07 noise reduction foam.
Try to isolate the noise problem and then replace that part(s) if it's viable.
If it’s your case fan there are fans available with manual speed settings and others with lower rpm's.
This can make a huge difference. If you can get one with manual speed setting outside the case like the Antec 120mm TrueQuiet.
That fan has silicon grommets but you can also partner up other fans (that dont have this) with the vantec vdk-120 system fan vibration dampener kit (this is relatively cheap).
(If you are concerned your pc will run to hot then you can run Intel or AMD's diagnostic test (depending on your system naturally) to gauge your system temp. You motherboard might also have come a disc containing system monitoring utilities.
Look at another cpu cooler. Stock cpu coolers are usually noisier. They usually give the dBA rating the cooler's specs.
Run your pc without the graphics card to see if that's the problem.
My previous graphics card was hella noisy and i replaced that.
Some graphics cards come with dual dual fans and others one big fan. Other with "true copper" and other with bigger heatsinks.
The brand will usually brag about it a lot so you'll find this info on the product page.
If you dont do hard core gaming one can also look at the silent heatsink gpu's like the
http://www.sapphiretech.com/presentation/product/?cid=1&gid=3&sgid=1088&pid=1240&psn=&lid=1&leg=0
If you think it's your cpu cooler and is certain it runs at unnecessarily high speed you can try a cheap solution such as the.
Zalman Silent Connector- ZM-RC56