FWIW: The human body, after all, is a machine and machines make noise when they function.
I remember sensing "noise from within" as I lay in bed (around 5 years old). I asked my Mother about its source and she had no answer.
I am hearing this same noise as I write and deem it "residual noise from electrical activity in my body".
I think Tinnitus must be this noise on steroids.
BTW, does anyone hear perfect silence?