Purring is reportedly very complex and often misconstrued

The Diary of Lupin Pooter Tygers, Tygers!
Marceline grooming Bubblegum. Both cats seemed content, but neither was purring.

I’ve only ever heard a cat purr when being brushed or stroked (aka petted) by a human being. Certainly that’s the only circumstance under which any cat I’ve owned or cared for has purred, but the BBC says that a cat may purr for multiple other reasons, even when it’s shuffling off this mortal coil.