Historical clue formulations

• Steady engine sound — 2024-12-26
• Hum monotonously — 2023-11-01
• Engine sound — 2016-09-24
• Play lazily, as a guitar — 2013-10-08
• Dull tapping sound — 2013-04-24
• Rhythmic humming sound — 2010-11-30
• Tap idly with the fingers — 2000-09-24
• Guitar sound — 1998-08-09
• Recite in a monotone — 1995-02-10