When it comes to tuning into the felt sense of metta, it’s common to try not to fool oneself by 'imagining' sensations.
We think that there’s a truth about what sensations are in the body, and we try to become like a better microscope, with a finer sensitivity to what those sensations actually are.
As common-sensical as this view seems, it can actually be a stance that hinders the development of metta.
It’s not like there’s a perfectly formed sensation hanging around somewhere “in the body” (or “in the brain”) waiting to be perceived. Perception is not a passive process, like a window which merely lets light through without altering it; it’s active, constructive. There's no such thing as a purely objective sensation, and therefore no ultimately ‘correct’ way to view them.
For instance, sometimes what at first seems very much like static, solid pain, after closer attention is paid, can dissolve into a bunch of changing flows of sensations; pressure, tingling, warmth, sharpness, ache, etc. and some of these flows are unpleasant for moments at a time, and then not. So where did that solid pain go? Well, it wasn't a real 'thing' in the first place. It's partly dependent on your expectations, your way of looking at or interpreting those sensations. Athletes are good at experiencing fear as excitement. Similarly, you can lessen pain by choosing to interpret it as pressure. See this video about an experiment with arthritis for a similar example. They use a visual illusion to make it appear as through people’s fingers are stretching while pulling on them, and it eases the pain quite effectively.
Experience is constructed, and you can construct it differently depending on how you relate to it. This is true also, of course, of the felt sense of metta. That warm glow in the chest (or however metta shows up for you) is neither “made up” in the sense that it’s not really there, nor an objectively true sensation; it's interactive; it arises from your body and nervous system being a certain way, and a certain way of relating to it.