A few months after I got Nebbie, I kept thinking I heard a truck idling outside. Every time I went to see what it was (figuring it was some sort of delivery), there would be nothing there. Eventually I figured out it was Nebbie purring!
Paizly has a very quiet purr, most of the time. Only when she's settled down and TOTALLY relaxed (usually at night, on my bed, and Nebbie's not around), does she get loud enough for me to hear it, rather than feel it.