My Almera N16 which has a QG18DE has a similar problem and when the mechanic took off the rocker cover the outside of the timing chain had rubbed flat and a groove had been formed on the case near the front cam sprocket. The cause he told me was the timing chain has stretched. A new timing chain kit ordered and fitted, a service since the fluids had to be drained since it was near it's service period and my motor is purring like a kitten.