This happened to my ute after I got it lowered. I did some reserch and found that after driving in the rain and then letting the car sit. The leaf springs build up 'rust dust' in between the springs which creates the creaking. Just wash them out really good (in between the springs) and you should spray all of that built up rust dust between them and will prevent the noise for a while, obvioulsy it will always build up after a while so you just have to give them a spray every now and then.