Due to the proliferation of cellular phones and walkmans it's not quite common in our days to take care of a quiet environment. Anyway I want to give some recommendations for the polite ones.
Computer noises are caused by hardware (fan, optical drive, hard disk) and applications.
The beeping of X11 windows can be configured to a shorter and lower pitched tone or even to a blunt "thump" with xset b ... options (a lower pitched tone is usually less annoying and distracting). Independently of that, most xterm-compatible windows and shells can be configured to make "visual bell" instead of "audio bell". For the console setterm -blength 0 and for X11 xset b off turns the bell off. See also the PCMCIA-HOWTO and much more details in the Visible-Bell-Howto.
When starting your laptop with PCMCIA-CS configured correctly, this will be shown by two high beeps. If you want to avoid this put CARDMGR_OPTS="-q" into the PCMCIA configuration file, e.g. /etc/default/pcmcia for Debian/GNU Linux.
To avoid the dialtones during the modem dialing add
module "serial_cs" opts "do_sound=0"
to /etc/pcmcia/config.opts (from man serial_cs). This will disable speaker output completely, but the AT M command should let you selectively control when the speaker is active, e.g. AT M0 turns off the modem's speaker.
Please make sure what you are doing, when configuring the fan. Your laptop may overheat and die, in case you have done something wrong. Just in case you want to check the fan try to cause a heavy CPU load, for example by issueing md5sum /dev/urandom. Now top will show an increased CPU load and the fan should began to run eventually. Note: usually you need to have been connected to power, otherwise the CPU might reduce load by itself. Also watch for the CPU temperature acpi -bt or cat /proc/acpi/thermal_zone/*.
For some laptop series there are Linux utilities available to control the fan and other features.
Toshutils by Jonathan Buzzard for some Toshiba models.
tpctl IBM ThinkPad configuration tools for Linux by Thomas Hood.
i8k utils for DELL laptops.
With some laptops the fan is always on or at least very often. Here are some remedies.
Sometimes the parport causes the fan to be always on. You may edit the /etc/modules.conf to cure this:
alias parport_lowlevel parport_pc options parport_pc io=378 irq=7
To avoid unnecessary hard disk noise you may use the same techniques as described in the power saving chapter above. Modern laptop and notebook hard drives come with a so-called "Acoustic Management", just have a look into the manual to get an overview about the possible settings.
Some hard disk manufacturers offer dedicated tools, e.g. Hitachi's Feature Tool allows to change the drive Automatic Acoustic Management settings to the Lowest acoustic emanation setting (Quiet Seek Mode), or Maximum performance level (Normal Seek Mode). Also hdparm -M offers some Acoustic Management options.