Most of the noise emitted by a computer is produced by the fan, the harddisk and the speakers.
libsensors0 is a library to read temperature/voltage/fan sensors
lm-sensors "Kernel drivers to read temperature/voltage/fan sensors. This is a module for reading the temperature/voltage/fan sensors in Linux via the LM78/79 chip and possibly sensors on the SMBus (System Management Bus, usually found in P6 and P-II systems). The LM80 and a LM78-clone called W83781D are also supported."
http://www.lm-sensors.nu/
The noise of the hard disk can be very disturbing, see man hdparm to reduce the spin of the disk.
For the console setterm -blength 0 and for X xset b off turns the bell off. See also PCMCIA-HOWTO, and much more details in the Visible-Bell-mini-Howto by Alessandro Rubini.