Sometimes, when Mousie was laying there, exhausted from taking a nap, eating, washing etc., she would do this half purr/half meow that kind of sounded like a hum. It went like this: MMMMMMmmmmmmmMMMMMm. Like she was kind of moaning, but not in pain, and at the same time wanted to meow, but couldn't motivate herself to open her mouth. It was like a hum - but pathetic! At the end of making the sound, she'd sometimes partially open her mouth and go, 'mah'.
Do any of your kitties do this?
(pic taken 2015)