A machine’s steady sound can hide a change that an attentive operator hears immediately.

Someone who has spent years beside a machine is not listening to it in the way a visitor would. They have an internal recording of how it should sound, and what reaches them is the difference, not the noise. To anyone else the room is simply loud; to them a bearing has changed. This kind of knowledge cannot be written down easily, which is why it is undervalued up until the person holding it leaves.

Wherever you work, spend enough ordinary time with a thing to know its baseline. And if someone tells you something sounds wrong without being able to say how they know, take it seriously and look.