This may sound stupid but my bike did the same thing. It ended up being one of the wires behind the number plate squeaking on the frame. Remove your number plate and check the wires and cables first. If that gives you no luck then the steering stem bearings may need to be lubricated. As for your forks, they do not need to be lubed because they are full of oil on the inside. You have a low maintnence bike, keep the oil changed and the chain lubed and that bike should last you many years of service.