Shadow also purrs constantly. She's loud, too. I think our neighbors down the street can here her.

Callie is a silent purrer. She does drowl (I think it comes from her mouth???) though when she is getting a really good scratching.

Whisper is a silent purrer but he makes "mrrreow" sounds when he is purring. I think him and Callie are both broken--they need new motors!