My Lady does that too
When shes comfy and warm and totally relaxed..she "purrs" too..lol.My husband used to call me over and ask me if she was ok,shes ALWAYS done this.She does it when shes relaxed and cosy,I think its really sweet
Shes not as loud as my cat though..my cat sounds like a harley! lol.But I would deffinately call it a "purr" as crazy as it sounds