The humming sound
I live in an apartment and some days I notice a faint, pulsating humming sound. Sometimes I wake up early in the morning and that sound prevents me from snoozing some more. It's just a faint droning noise that lasts hours at a time. I know I'm not crazy and I know it's not tinnitus because I know the sound is coming from outside somewhere, because it grows noticeably louder when I open my window. If I plug my ear this also blocks out the sound.
Any ideas what the source of this humming sound is?
Does anyone have an idea what that sound is coming from?