As the title says. Is there anything I can do to try and eliminate it before I take it back to Decathlon? Everything seems to work fine apart from that.
When you push the bike backwards you are forcing the gearbox and motor to turn backwards.
Which it's designed to be able to do, but you will be putting unusual stresses on the gearbox and motor so it may well creak.
Try spraying the motor unit and casings with silicone lubricant.