Here's how it works. You put the usual binaural "ear pieces" where they'd normally go (if you put them elsewhere, I do NOT want to ever use your mechanics stethoscope). Then, when the rattle is happening on the car, you simply place the rod onto various parts of the engine or components - making absolute certainty that you do NOT get it stuck in moving parts such as fans or belts. You will hear a slightly amplified form of the rattle depending on how close to the rattling part you really are.
Here's my experience. I went out with this to check the loud rattle on the civic. Since it was cold (14 degrees), I put the ear pieces to my ears before I went out to the car so I could also put on the balaclava. I'd suggest NOT doing this - every time I bumped the bottom end of the stethoscope, there was a really loud "thump" that seemed to cause pain. Once I had the car started and the hood up, I began methodically setting the rod onto various components. The engine block was first (new engine, wanted to make sure it was okay), transmission was next (wanted to make sure there wasn't a bad bearing or torque converter going on, or a stripped gear), and then the usual suspects. It all stopped with the alternator. It seemed to be the loudest. The tool seems like it worked well. I needed to remove the alternator to get it checked.
Here's the result. I titled this post "It's a Dangerous World Out There". So where was the danger? Aside from not getting the thing caught in any moving parts, what could possibly have happened? Well, I identified the alternator as a potential problem (and then saw the pulley on the alternator sitting at a bad angle). So, I decided to remove it. While I was laying underneath the car, clothing starting to stick to the concrete beneath me, with a large wrench to remove the alternator bolts, and not having much feeling due to the cold at this point, I dropped the wrench. Normally this is not a problem. However, I've been in the cold for a bit so the bridge of my nose is numb (right where the wrench hit), and I'm already frustrated. Luckily, I was wearing glasses, which broke the fall of the wrench. I rolled out from underneath the car, bent the glasses back to shape, felt kind of odd about my nose (no blood from the inside, so it's not broken), and went back to work.
Every time I stood up, I felt a little weird. When I finished up as much as I could (the alternator is still there and needs to be disconnected), I went inside to console my sweet wife (another failure). Looking in the mirror later that weekend told me I did break the skin. Apparently, it's hard to take me seriously when I've been hit right between the eyes.
