The creaking sound is usually caused by the screws in the neck becoming loose.
Tighten the screws above and below the folding point of the scooter regurarly with a hex key.
You can control the tightness of the locking mechanism by tightening the adjustment screw. The adjustment screw is on top of the silver lockinghook, which is found in the front of the scooters neck.
The locking mechanism is adjusted correctly, if and when, you can lock the neck by hand.