Hearing a heartbeat while lying down is normal, especially if you're on your left side, as the heart rests on the left side of your chest wall. Moreover, when you're stressed, you become more conscious and irritable with any external sounds, and it becomes more acute. However, pulsatile tinnitus, a repeated beating sound in your ear, can also be a cause and shouldn’t be ignored as it may underlie a serious medical condition. Seek medical attention for more evaluations.