I have 7 cats and there are three that sound like diesel engines. Sometimes at night when they're sleeping with me, I put my head down on their belly and the purring helps me fall asleep. That's the one thing I love about cats.
All my cats were loud purrers as kittens, but have gotten quieter as they got older. I still love to press my ear to their chests and listen to them purr. It is so comforting.