One of my observations is that humming a note within my valid vocal range has "feeling" differences even within the hard cutoffs (usually about 2 octaves for most people). But those cutoffs (and presumably the feelings) move between childhood and adulthood.

(As an adult, my perfect pitch is not completely stable - if I don't use/tune it regularly, it can drift up to 2 semitones, but no further regardless of how long between tunings. Unfortunately I never did tests as a child.)