When traveling on DC9s and, I think on MD8-whatevers, there is this quite loud tone, or howl heard during the announcements when on the ground. It sounds like "rrrr Ladies and gentlemen rrrr welcome aboard Delta Flight 89 rrrr" and so on. I am not able to discern the pitch, but it sounds higher than the 400cps* of the on-board AC power. First, what is it, and second, why has no one found a way to isolate this annoyance? In many cases it overpowers what the stew or the pilot is trying to say.
* "cps", or, cycles per second, is Hertz if you are over 55 years of age.