How does a pipe organ make noise/sound?

A pipe organ makes noise/sound when the column of air enclosed in
the pipe is disturbed and is set in vibration. Before
electricity was harnessed, someone with a set of bellows would
supply the air. Today, the same effect is accomplished by
electric rotary blowers which deliver a steady amount of air
under constant pressure.

In order to produce a variety of tones, most organs have several
sets of pipes, called registers or stops, set in a "wind chest."
When a person presses a key, a valve in the register beneath the
correct pipe opens and lets a stream of air into the bottom of
the pipe. This vibrating column of air, whose molecules move up
and down parallel to the length of the pipe, produces the sound.
Also, the longer the pipe, the lower the note.