Computers only work well in mild, stable-temperature environments. If the temperature swings wildly between daytime and evening (as it is apt to do in a garage), the computers will fail rather un-gracefully. I do not recommend that you keep them in the garage, attic, or anywhere that people would not be comfortable.

Instead, I would recommend doing what I do in my office: Keep things turned off that aren't specifically in use at that moment. Our print/file server and the attached printer only gets turned on when it's being used, for example.

As far as noise of running components, there are options for reducing the amount of noise made by your computer's power supply and fans: http://www.pcpowerandcooling.com/