There are several posts at the site on “niramia sukha“. It is NOT a feeling (vedana) in the sense of a sense pleasure; it does not belong in the vedana cetasika. It is a “sense of relief” when one starts realizing the “burdens” associated with sense pleasures.
- The closest analogy (if one can be given) is the feeling one gets when one is relieved of an headache that one had since birth.
I have made a single page chart that shows how the niramisa sukha starts as one starts when one first hears the true message of the Buddha and starts comprehending it, and becomes a Sotapanna Anugami. Then it makes permanent stepwise jumps at the four stages of Nibbana culminating in the Arahanthood.
The pdf file can be saved or printed for reference:
Discussion of the chart is at: