He could just be very excited, and that's how he expresses his excitement. Is your other cat around when he does this? Perhaps he's defending his "catch" against anyone who might try to steal it from him. The best defense is a good offense, after all. One of my kittens used to growl the most hilarious (and surprisingly loud) kitten growl when he snagged a highly desirable toy, and he'd run off with it to enjoy the fruits of his labour away from his littermates (an exercise in futility). He was also the runt of the litter, so a bit of small-man-syndrome was probably a factor. If your little guy is otherwise healthy, I wouldn't worry about too much.
One other thing, do these toys have catnip in them? Usually catnip doesn't effect kittens much if they're under 6 months, but that isn't always the case. Maybe he just can't handle his drugs yet.
