Has it ever happened to you? Streching your syllables, turning "I" into "Iiiii...", punctuating your sentences with a few 'thoughtful' "hum..." while searching for your words?
I have once been told by someone that I talked to much. That it showed I was nervous. It's the exact same thing.
There's no need for that. No matter how we fill the silences in your conversations, we don't need to.
First of all, it's annoying as hell to hear. Hums, strecthed syllables... they just pollute your speech. They distract, make noise, for no reason.
But, more importantly, it gives off the feeling that silence makes us uncomfortable. That we need to fill it, to hide the fact that, right now, we don't have the answer.
On the contrary! Keeping quiet, taking time to think about our reply, to build it, is a positive signal.
First, it shows you believe this question, or the topic, merits a real answer, and not just something off the cuffs.
Second, it tells the person you are speaking with, or the public, that you are taking the time to find the answer you believe the question deserves.
Finally, it tells the world that you have enough confidence, self-assurance, to expose your ignorance, your doubts, and your limits.
When we don't know, when we need to take some time to consider our answer, to build it, we own it.
We own it, we shut up, and we take the time it will take.
It's not an issue. It's not a sign of weakness.
On the contrary. It shows thoughfulness, respect, and self-confidence.