I believe there are two aspects in this post.
1 - Physical feelings during Metta.
2 - Parts of the body where a specific type of sensation is felt.
In my experience of Metta meditation the origin of the warmth feeling associated with this type of meditation is usually the centre of the chest. With a deepening of concentration the feeling expands and takes over the whole body.
When meditating, Metta or otherwise, feelings of tingling, energy or "electricity" in the body are sensed in many different parts, but in my case tend to focus on the back of my head, on my forehead, on the chest or in the base of my spine.
If when trying to develop Metta I have a stronger feeling in my forehead, I tend to consider it a distraction for the type of meditation I propose to do. If I am doing some vipassana exercise I focus on that feeling and try to investigate it.