The Pook, you and Faldage appear to be talking at cross-purposes.

I did not ask for a word having the same effect as "Pfft!", though if I were, "Balderdash!" would be fine. My question is, what is the word describing that sound. It is an optimistic question, but the infinite English language has given us technical terms for all sorts of strange noises emanating from the body by design and otherwise. You seem to be hung up on the idea that the term describing a sound must mean the same thing as that sound. If some one tells a joke, you do not shout, "Laughter!" The word "laughter" describes the staccato sound you make in response to something funny.

So, let me rephrase: Laughter is to "Ha ha ha!" as X is to "Pfft!" What is X?