Place to put a stethoscope

CHEST

Listen closely—this body part houses the heartbeat you’re trying to monitor.

A stethoscope is typically placed on the chest to listen to the heart and lungs, so the answer is the body part where doctors position it. The clue is straightforward with no wordplay—it directly names the location where the tool is used. If you think of where a doctor would press the stethoscope’s diaphragm, "chest" is the obvious fit.