I have an adolescent cat that I adopted as a kitten (not this one :P ), and he growls when playing with his toys. At first I thought that he was growling at our other cat (also adopted as a kitten) for getting too close and trying to "steal" the toy but it quickly became obvious that was not the case as he would growl when he was all by him self in the other room.
The cat's history before his rescue is unknown to me, but I do know that he was rescued from a very traumatic situation being stuck in a road gutter for several hours.
He only growls at the toys made of "fluffy" material, never at the "non-fluffy" smoother ones. He's never growled at me or the other cat (that I know of).
I once attempted to get really close while he was growling just to see if he was in an aggressive mode and he was quite calm even when I reached out and took the toy he was playing with.
Is this normal behavior or just some sort of personality quirk?