Historical clue formulations

• Contented sounds — 2026-05-21
• Runs smoothly — 2019-12-22
• Engine sounds — 2018-12-13
• Sounds from a happy kitty — 2016-08-30
• Contented cat sounds — 2015-08-10
• Hums — 2011-09-21
• Cat sounds — 2008-09-15
• Engine hums — 2005-05-10
• Runs smoothly, as an engine — 1998-12-28
• Doesn't knock or ping — 1998-09-19
• Sounds of contentment — 1998-03-08
• Good engine sounds — 1997-11-26