…chanting involves the activities of the upper and lower lips as well as the tongue. All three must be engaged in chanting the Hare Krsna maha-mantra. The words "Hare Krsna" should be very distinctly pronounced and heard. Sometimes one mechanically produces a hissing sound instead of chanting with the proper pronunciation with the help of the lips and tongue. Chanting is very simple, but one must practice it seriously.

From Cc Adi 17.32 ppt., by Srila Prabhupada