Possible Clues:

• Sounded content
• Reacted to head scritches, maybe
• Made contented sound of cat
• Sounded like a new engine
• Sounded contented
• Showed appreciation for being scratched behind the ears, may
• Made sounds like a shorthair
• Ran smoothly, as an engine
• Vocalized feline bliss
• Sounded contented (when stroked?)