My dog growls when we play tug, is this bad?

Hi, I have watched both The Power of Playing Tug with Your Dog and Advanced Concepts in Motivation. I am a couple of weeks into implementing the techniques with my own dog and I have a question about growling--is it good or bad that she growls when we tug? If it's a bad thing, how do I address the issue?

When I release the tug and run away from her she comes charging back to at me--she will even jump towards me with the toy in her mouth. She growls while we are tugging, while she is jumping at me with the toy in her mouth, and when I hold the toy steady.

I don't remember Michael Ellis covering this in the DVDs, but if he did, could you tell me where to find it? Or perhaps there is a relevant article or Q&A that I overlooked on your site. Thank you very much for all the help. The DVDs are great! Worth every penny.