Vibration motors are perfect as non-audible indicators. Use in any number of applications to indicate to the wearer when a status has changed. The unit vibrates softly but noticeably. (adapted from Sparkfun)

// Prepared by T.K.Hareendran
// Tutorial link: https://duino4projects.com/how-to-play-with-pancake-vibration-motors/

void setup() {
  pinMode(13, OUTPUT); // Initialize D13 as an output pin
}

void loop() {
  digitalWrite(13, HIGH); // turn the drive on (HIGH)
  delay(500); // wait for 500ms period
  digitalWrite(13, LOW); // turn the drive off (LOW)
  delay(500); // wait for 500ms second period
}

• https://www.precisionmicrodrives.com/vibration-motors/coin-vibration-motors/