Dales Room Key->Hum Sound
Let's remember where that key has been for 25 years. Yup, in the black lodge in Dales pocket.
I'm pretty sure that Key is where the hum sound is coming from.
The hum started sometime last week. I don't think the key couldn't have arrived that long without Beverly notifying Ben sooner.
My first thought was those secret passages Audrey used to spy on people. Maybe an object is in there that's making that hum.
But I don't know.
If you look closely at the Owl Cave map drawing: could there be an underground tunnel from The Great Northern to the Red Room/Lodges? After all: badCoop/evilCoop/the-later-to-become Mr. C became conscious again in room 315, right?
Is it the same hum you heard in coopers first dream back in season one when the arm is rubbing his hands together?
Thinking about it dale was 25 years older in that scene so is that sound just resonating now.