Many - if not most - dogs growl while they are playing tug-of-war. Is that what is happening? *IF* that's the only time he growls and he doesn't try to guard other objects (like his food or a bone), then I honestly don't think you have anything to worry about. It sounds like play to me, too.
Just watch for a little bit, but otherwise, don't worry.
__________________  FirstYorkie
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