I have two cats- 8 month old litter mates, one male and one female. Over the last few months, the male has started growling whenever I give the cats new toys. He usually takes the toy in his mouth, runs under the bed, and growls loudly. If the other cat comes near, he will sometimes hiss. I thought it might be related to toys containing catnip, but he does it to both catnip-scented and "drug-free" toys. I usually take the toy away after 10-20 minutes of growling. If I give them each a toy, he will take one, put it under the bed, then go take the other and growl under the bed with both. Sometimes he comes out and sits in random places with the toy in his mouth while growling.
Any suggestions for discouraging the growling? He does not usually growl otherwise and there is no aggression between the cats. I'm hesitant to give them new toys now because I don't want it to end up in a fight.