@MrMoody - I prefer the computerized version of the technology myself. You can buy dynamometer software for your car to get actual readings on how the engine is revving.
That’s better than simply listening for strange sounds to determine if the car is operating normally.
I remember I had a problem with my Honda where it would idle high when I was at a stop light, and rev back and forth, making a whole bunch of noises.
It was embarrassing because it made it sound like I was trying to race the guy next to me! When I drove normally, it would rev fine. I wish I had had a dynamometer software application at the time, so that I could keep continual recordings of how the engine was actually performing.
Then, when I brought it to the mechanic they would have actual data to look at, rather than hoping they could duplicate the same condition in the shop, which they could not always do sad to say.