Couple of things...is your rescue the younger of the pair? If so..all above have it right..she is just playing...maybe testing the grounds for what she can get away with. ( Awwww heck..I think they all test regardless of age, but younger dogs seem to go at it till they get their correction form the other dog )
Secondly, Ashley and her boxer friend played a few days ago, and when he had enough..he pinned her for a split second..showed his teeth, and did this little growl that lasted for maybe a half second. My neighbor ( he is her first Boxer ) was REALLY scared that her boy was going to hurt Ashley..I tried to explain to her, corrections are needed, and human intervention is to be discouraged ( of course with the exception of a full blown FIGHT..but you will never mistake THAT for anything than what it is...) Yes..they are loud, they sound like they are a full pack of wild animals going at it..but its all in play for these babies....he will let her know when enough is enough...just don't get yourself stuck in the middle!! OUCH body slam by two oncoming boxers will not be fun..and your skin may tear alot easier in the case they are showing teeth

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Good luck..they will sort it out soon!