When you click and hold down on a ball, it will vibrate.

VibratingCircles.pde

// Array to hold ball objects
Ball[] balls = new Ball[int(random(8, 18))];

void setup() {
  size(600, 600);
  noStroke();
  
  // Creates balls with random properties
  for (int i = 0; i < balls.length; i++) {
    balls[i] = new Ball(random(width), random(height), int(random(80, 120)), random(1, 5), color(random(255), random(255), random(255)));
  }
}

void draw() {
  background(0);
  cursor(ARROW); // changes how the mouse looks
  
  // iterates through the array of balls and executes
  // the code in the for loop for each ball
  for (Ball ball : balls) {
    // checks if mouse is over ball (see how it works inside Ball class)
    if (ball.hover()) {
      cursor(HAND);
      if (mousePressed) {
        ball.x = mouseX + random(-ball.intense, ball.intense);
        ball.y = mouseY + random(-ball.intense, ball.intense);
      }
    }
    ball.display();
  }
}