This is quite normal with wide low profile tyres all the Ghibli 3`s I have owned have done this in cold weather.
You will find in warm weather it doesn’t do this as the side walls of the tyres have more give in them.
I fitted a set of 18 inch wheels with snow tyres and the the noise went away as the tyres have a larger profile and more give in the side wall
I just bought a load liner from an American website , Fits perfectly to the boot floor. If this is of interest to you let me know and I will forward you the site address. I think it was approx £ 175.