Fascinating. I used to do decades ago with my ears what their devices do now: Listen to machinery while walking around the plant I worked in to see if I could pick up on a trouble spot, because I learned over time that mechanical equipment that was heading for failure sounded a bit different then the equipment that was still running smoothly. Belts, bearings, rollers and plates would wear and I would know to pay more attention to them more often, rather than just react when something happened. People thought I was nuts when I told them I could hear that something didn't sound right through all the noise, which was considerable (you had to wear hearing protection all day).
Some of the more fancy machines these days have embedded microphones to tell if a bearing is going to fail soon. They make a distinctive noise based on shaft rpm and bearing diameter when they start to fail, so you know which one to order before you spend a day tearing the machine appart to get at the faulty bearing.
Bearings also tend to start running quite hot when they are at the end of their lives. Bearings on trains often have smoke bombs on them that go off at a specific temperature.
