alarm bell; warning bell
A bell rung rapidly to warn of danger, such as a fire or attack. Often used in the phrase 早鐘を打つ (to ring an alarm bell) and metaphorically for a pounding heart.
The alarm bell rang out to warn of the fire.
The pounding of my heart wouldn't calm down.
Compound of 早 (haya, 'fast') and 鐘 (kane, 'bell'), referring to a bell rung quickly as an alarm.