Your device is not faulty – the sound is due the Camera Lens Barrel required to operate the Auto Focus function.
Your Galaxy A8/A8+ 2018 supports a 16MP main camera and it supports the Auto Focus feature required for high quality and high resolution of camera. Auto Focus feature adjusts focus by moving the Lens Units up and down over the Image Sensor. This enables Auto Focus speed and accuracy and helps in reducing battery usage.
However Camera module is fitted with a “Stopper” part to limit lens unit movement within the space for the Camera module.
When the Lens and Stopper touch, when shaking the Galaxy A8/A8+ 2018, a minute noise may occur, however this is normal due to the operation of the Auto Focus feature of camera.