Example aspects of a nozzle cap for a fire hydrant and a method for manufacturing a nozzle cap to detect leaks in a fluid system are disclosed. The nozzle cap for a fire hydrant can comprise a cap body, the cap body comprising an inner housing and an outer housing, the outer housing defining a cavity; a vibration sensor received within the cavity and configured to detect leaks in a fluid system connected to the fire hydrant; and a metal insert contacting the vibration sensor and the inner housing.