[magnaton]

Yes indeed. In extreme cases, it can give the impression of a xylophone! To control chiff, we use a pointy double-sided knife called a "nicking tool" to create a series of evenly-spaced nicks across the entire width of the flue (which is the slot at the base of the mouth, formed by the lower lip and the languid inside the pipe; air leaving the foot of the pipe is focused by the flue on its way towards the upper lip). Often the nicks are cut into both the lower lip and the languid at similar depths and perpendicular to the flue, although there are exceptions; the width, depth, and spacing of the nicks all have tonal implications. Care must be taken to avoid over-nicking, as the articulation -- some of which is necessary even when chiffing is not desired, and which may need to be more prominent when close to the pipe than when listening from the room -- can be completely lost.

[Footkerchief]

Fascinating. This is the most scientific explanation of nicking that I could find:

http://www.colinpykett.org.uk/physics-of-voicing-organ-flue-...