He is probably talking about a stethoscope you can purchase at most auto parts stores. For mine, I took an older medical stethoscope I had and inserted a 24" stainless steel rod on the end.
You are able to insert this into a space, place it against any surface and the sound is transmitted clearly and cleanly to your ear. Same principle as putting a glass against a wall and listening to a conversation on the other side.
Note: If you don't have a medical stethoscope or access to a store that sells them, a simple dowel rod will work. Round the ends and place in your ear...it will give you the same results for a fraction of the cost.
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