When you know your value, you are comfortable with yourself, we say you are confident. Confidence does not need to boast. Because if it boasts, then that is confidence in over-drive, or over-confidence. Confidence is quiet, it is silent. It knows it's worth, so it rests in its inner knowledge.
When you have deep insecurities, you suddenly feel very anxious. Now, you can't keep silent. You need to share your fears. You need external help, counselling, and reassurance. So you keep speaking. Insecurities are never silent, they speak.
Wisdom is silent. Because it knows. It does not seek outside. Because it has found the answer within. Wisdom wears the clothing of confidence. Hence, confidence is silent.