Grooved...
I had a what I thought was a rattle when I fitted my AKS BBK, came with 370mm slotted/ grooved discs. Bugged me for weeks, I had Alex demented, convinced there was a rattle
until after lots of research, discovered it was air being pushed pass the slots creating a kind of rattle sound.
I don't even notice it now, I just think you're always a bit sensitive to new sounds