You know the sound. It's the sound we make when we're indicating that we're hearing words, or at least sounds, but we're not comprehending a damn thing. It's the sound made by all the "Peanuts" comic strip adults, but most famously by Linus's beloved teacher, Miss Othmar.
Remember Miss Othmar? She later married and became Mrs. Hagemeyer, but Linus and pretty much every other "Peanuts" denizen called her Miss Othmar still. (I have no idea why Wikipedia names "Mr. Hagemeyer" as her EX-husband, as she never got divorced in the strip. Probably just some Wikipedian having fun, or being confused about the name thing.)
I forgot totally about this, but apparently Miss Othmar was fired in a 1969 strip after a teachers' strike. Pretty bold move for Charles Schulz.
The Peanuts Wiki has a little more info. I forgot that Linus gave her eggshells for a wedding gift.
That Wiki also notes: "Miss Othmar was voiced by a trombone, as were most of the adults." Heh.