If you imagine that instead of air molecules, you were flying through glass marbles - (yes I know, bonkers, but stay with me here!!) - there would be a hell of a noise in lower levels (full density marbles), with billions of them all hitting and bouncing off all of the windscreens and structure. But, higher up, owing to the much lower density, there would be far fewer marbles hitting the windows etc., (even though, at a constant IAS they would be hitting at the same speed), so it would be much quieter.
Does that help?
PS, the Airbus FBW avionics and cockpit instrumentation cooling also makes a hell of a noise, but its mode changes as the outside air temperature reduces, so that noise reduces with height as well.