I've noticed a semi loud droning sound lately. Like as if its rotating at a linear rate, not with rpms, tires, etc. Its a humming sound. Kinda like the sound the metal makes on submarines in the movies, but in a hum (I really have no idea how to describe it, and my camera doesn't pick up the sound). I can feel a slight vibration from it through my gas pedal, or Im just imagining things. It's present when cruising and engine braking; that's when I notice it most. 2009 Cobalt LT, 90,000 miles. Any thoughts? I was thinking maybe the torque converter since the sound is on and off in a slow linear pattern, but I have no idea, to be honest.