I've had many cats who whirred or trilled- but we recently finally caught a very smart feral momma-cat named Sophie, after catching three litters-ful of her kittens without being able to get her- and ALL of her family members do that, to an amazing degree- it's like a special language she and her children have all to themselves, when it's mealtime it sounds like a roomful of PIGEONS or something here! But it's a very happy sound, and they do make it a lot when they're getting ready for a good treat or at mealtime or when they're charging around the house at Mach 2 in the middle of the night- it's a very pleased, contented kind of sound and I'm glad they make it-