I get asked a lot as a trainer from students if it's normal for their dogs to groan and growl while playing. It is normal. Dogs vocalize quite a bit while playing. All normal. Always make sure their bodies are lose and relaxed, no hard staring, fear or stiffness.
Here's our guys goofing it up with lots of vocalizing.
Here's our handout called Your Pit Bull and Other Pets that might be helpful. http://www.ourpack.org/otherpets.html