Towercase ressonance (on pee cees)

[Ilwrath]

Yep....  Tighten down everything you can, and maybe add some rubber seals or something to cut down on the vibrations.   Also, I've found using better quality fans (I run Enermax ultra-cools) helps quite a bit.  Most of the noise in my case is now my stupid chipset fan (which is failing) and the rustle of airflow noise.  

[Ilwrath]

I wonder if I could fit a motherboard inside an anvil...  

Or, you could just drop the anvil on the computer.  I don't think it would make much sound after that.  :-D

Of course, for serious advice...  I see people mentioning small fans.  Avoid those.  Go with larger fans at less RPMs.  Smallest fan in my computer (except for the malfunctioning chipset fan) is an 80mm case fan.  I run a Thermaltake copper CPU heatsink with an Enermax UltraCool 80mm case fan on it.  Better cooling and quieter than any other HSF I've seen.  And, if I want to overclock, I just turn up the fan on it, and suddenly I've got one of the highest rated heatsinks on the market.  At 3500 RPM, it managed to keep a severely overclocked AMD below 45C.  At 2500 RPM it has no problems with my P4 3ghz.  haha!!