Ben growls when we play tug with a toy, it sounds hideous but I know it's only playing, like Archie Bean says there is a clear difference in sound and body language.
Harry on the other hand is also a resource guarder, and he starts off play growling, but sometimes gets too wound up and it tips over into being guardy and a bit temperamental, but again you can see the difference and I always can see the signs and stop the game before it gets to that point.
You soon learn what the different growls and noises mean, and you know your dog better than anyone so watch his body language and you should be able to see if it's part of the game, cockers are so vocal though they do like the sound of their own voice