I think it was his eye! yes, it was this! He had the eye of a vulture ,a pale blue eye, with a film over it.
"every night, about midnight, I turned the latch of his door and opened it --oh so gently! And then, when I had made an opening sufficient for my head, I put in a dark lantern, all closed, closed, that no light shone out, and then I thrust in my head...I undid it just so much that a single thin ray fell upon the vulture eye. And this I did for seven long nights"
thump thump,thump thump,thump thump!
"Villains!" I shrieked, "dissemble no more! I admit the deed! --tear up the planks! here, here! It is the beating of his hideous heart!"