Maybe its for a similar reason to why cats purr; vibration make brain feel different in a way that facilitates thought?

source im fuckin g insame

that's the sound of our brains venting the thinking gas

Maybe it also has to do with the human anatomy? Like, when people are thinking they probably have their mouth closed and maybe even purse their lips. The sound you can make in this pose is really just hmm I guess.

OK yeah, the next question would then be why we use certain facial expressions...

Sign language is even more universal (early hominids def would've had signs before proper spoken language).

If I hold out my arm, my palm towards you, you'd probably know what I mean.

Like this.