I had just picked up a rental car to use during a work trip, of a make and model I'd never driven before. Just as I pulled out of the parkade onto the road leaving the airport and the speedometer moved above the 10, the car shook and a made a sound that had me frantically looking in the rearview mirrors to see what piece of itself the car had just dropped on the road behind me.
Nothing. The road was clear, the car wasn't shaking or making any funny noises anymore, and was accelerating smoothly. Weird.
Just as I pulled off the airport road onto the freeway, and accelerated further, it happened a second time. This was really weird. Still nothing on the road behind me.
It was probably 5 minutes later, as I was driving around a 270 degree freeway ramp to get going the way I wanted, that I figured out what the noise and vibration was. Something not visible inside the rental car parkade, and not in my line of sight when driving the car due to keeping my line of sight on the road.
Impeccable timing on that thunder, Mother Nature.
I'm glad the funny noise wasn't a sign of anything damaged or about to fail, but I've encountered those conditions more than a few times. One reason I pay attention to them!
I can't always say what the problem is, but I have pulled operating equipment out of service just because "it sounds wrong". A centrifuge whose hum (rather, whose muted roar as heard through the hearing protection) wasn't steady but a cyclical louder-quieter-louder-quieter sound is not perfectly balanced. Next time I walked past it after reporting it, it was shut down and maintenance had its parts strewn all around them. I've reported multiple pumps to maintenance that "sound unhappy", which all so far have turned out to be a failing-but-not-quite-failed coupling between the motor and the actual pump, fixed before flow was lost.
So maintenance may not always like that I can't say anything more than "it sounds wrong", but if I catch it before "we lost flow", or worse, the equipment is damaged beyond what can be fixed by replacing a coupling, they have a chance to go see what it sounds like for themselves and fix it before it's a real problem.
The hardest "wrong" sounds to notice, though, are the ones where a sound goes missing. I can only remember catching one of those; I was sitting in the break room with the hum of the plant coming through the walls and I can't even remember what got my attention. I listened, trying to figure out what was wrong about the hum of the plant, because I just knew "it sounds wrong". That hum was right, that higher-pitched whistle was right, what was wrong?
Then I realized that all I was hearing was steady hum sounds. There was supposed to be a steady banging sound as well, of a big diaphragm pump, about 70 beats per minute. That, unfortunately, was not a matter of catching the equipment before it broke badly, but it was caught soon enough to shut down the process before the upset caused by the lack of that pump got bad and we busted our permits.