Those people do not inhale the air smoothly, instead they gulp the air as if it is in the shape of a ball, which action produces the knocking noise.
This can be corrected by changing the method of air intake from gulp into inhalation. Specifically, to inhale the air slowly through the nose, and exhale through the mouth and vocalize. If the trainee repeats the movement, the inhaling noise will gradually disappear. The secret is the inhalation method. It is indeed a persisting habit, however, if this method is adopted, the noise will disappear, and the speech will turn to be more beautiful and clear.
