Cats have all kinds of different purrs!
From what I've seen, anyway. There's the silent, contented purr you're talking about... my cats do that, if you put your hand on their neck or tummies, you can feel the rumbling purr but they aren't audible.
Then there's the purr that is so LOUD, it keeps me awake! Snickers is my snuggler, when I reach out & pet him at night he'll go from the silent purr to the really loud purr.
Then there's a really happy, trilling purr, which I've heard several of my cats do. When they are about to be fed or if I get out the catnip.