A mechanic's stethoscope works the same way as a doctor's stethoscope. It's designed to pinpoint problems in the engine when you cannot visually pinpoint the location of a noise. These stethoscopes are used by professionals and hobbyists for common problems, such as locating engine knocks, air leaks and bad bearings. If you like to work on cars, a stethoscope may save you time finding the location of a problem.
Pinpoint noise in the engine compartment. If you have a knocking sound, or some other inappropriate noise in the engine compartment, perform a diagnostic sweep of the engine compartment. Leave the car running. Without the stethoscope, start in one corner of the engine compartment and slowly move across the engine, listening for the general vicinity of the noise. Listen to the individual engine components in that region for internal noise (such as engine block, alternator, A/C compressor, power steering pump, and master cylinder) by donning the stethoscope and probing the part in question.
Check the the transfer case. If odd sounds come from the back of your rear- or four-wheel-drive vehicle, you may have a problem in your transfer case. Make sure that the brake is on and the vehicle is on a flat surface, with the wheels chocked, and the transmission in neutral. Do this while the engine is off and the rear tires are off the ground. Have an assistant spin the rear wheels by hand as you listen. Place the sound collector directly on the different sections of the transfer case and listen for unusual sounds.
Check for vacuum leaks or pressure leaks. You can also use the stethoscope to detect leaks in the vacuum hoses, brake lines, and other lines that are under pressure. Using an assistant to apply the brakes or rev the engine, test the areas in question one at a time until you hear a hissing sound.
Check your wheel bearings or brakes. While the engine is turned off and the wheels are off the ground, slowly rotate a wheel and listen for grinding noises.
