Thanks Rosie.
Are you sure the noise you hear isn't just me scratching my head? ;o)
Seriously though folks, this is a new one on me.
I do however have one idea.
Over time, slates or tiles settle in. Any slight gaps between each tile becomes plugged up with lichen/moss/dirt.
The new tiles (especially if they are concrete interlocking tiles, will kind of "sit up" until they finally settle under their own weight.
Throw in some dirt and moss etc, and all gaps become sealed, so to speak.
It might just be this. Wind whistling through the cracks :o)