Sibilance is a pitched hissing sound produced by some people. It occurs when the natural placement of the teeth and tongue create a whistling sound when pronouncing the soft consonants sh and ch, as well as z, x, f, and soft c.
A perfect sentence for testing one’s tendencies toward or away from sibilance is “Sneaky Susie snatched Sally’s soft satin shoes stealthily.”
If you hear this problem in playback of your work, you can teach yourself to eliminate it by doing the following: Get a thin stick, roughly a sixteenth of an inch thick (a Popsicle or ice-cream-bar stick will do the job nicely). Insert the stick between your upper and lower front teeth. Do not bite down on it or press it on your upper or lower front teeth. Hold it so that your teeth can come together and separate freely. The idea is to put the stick gently between your teeth to create enough of a space to stop the whistling. Over time, you should be able to capture a muscle-memory sensation that enables you to keep the space created by the stick without keeping the stick itself between your teeth, thus eliminating the sibilant whistle. If you find that over time your teeth gradually come together again and you start whistling, you should be able to consciously move your teeth slightly more apart without needing to use the stick. If you can’t readily do that, go back to using the stick. Over time, you should be able to avoid sibilance without the stick.