I think having a heavy metal heart means her heart is heavy... Like it takes a lot for her to feel anything because whatever has happened to her in the past. Her heart is "heavy metal" in the sense it feels stagnant and it takes a lot for her to really feel anything. She loses herself and her body begins feeling like a machine because she feels nothing.
Until the subject of the song comes in, they are enough to make her feel the pulse of her heavy metal heart. Weather it's a good or bad way, even while leaving them she can still feel it with just the thought of that person.
Just my interpretation, I might be totally off!
Also, "I can see a fish in the smudge, you'll be mine" is supposed to be "I can see a vision that's meant to be mine"
I think having a heavy metal heart means her heart is heavy... Like it takes a lot for her to feel anything because whatever has happened to her in the past. Her heart is "heavy metal" in the sense it feels stagnant and it takes a lot for her to really feel anything. She loses herself and her body begins feeling like a machine because she feels nothing. Until the subject of the song comes in, they are enough to make her feel the pulse of her heavy metal heart. Weather it's a good or bad way, even while leaving them she can still feel it with just the thought of that person.
Just my interpretation, I might be totally off!
Also, "I can see a fish in the smudge, you'll be mine" is supposed to be "I can see a vision that's meant to be mine"