vibrating bird sounds?

[candywishes]

Hi everyone, was having a cuddle with the piggys tonight and noticed when i was stroking duke and Gwen they'd kind of vibrate and make these bird like noises - hard to explain - know what i mean? does anyone elses piggys do this? does this mean they are happy?

 

[Joey&Gizmo]

Sometimes piggy purr when you stroke them and it sounds like a cat purr. But you say it was more of a vibrating sound so not sure what that is, unless it is there purr and all piggys purr differently?
One of my piggys purrs every-time i stroke him. :p