Beyond what's already been added, here's a thread with a description of a
Rottie Purr. My male only does it sometimes and it's usually after he's been asked to get some "rottie loving."
I've also found that when my male and female play together, they play rough and they play loud. The owner of the place I use for boarding, swimming, and now Rally training said her rotties play the same way.
My female is the more vocal of the two. Based on the training classes I've gone to and the trainers' comments, I watch their body language sometimes.
They do "play bows" to each other, each may roll the other, etc. My male will sometimes just lay there while my female dances around him, trying to engage him in play. That usually triggers my female to take off on a zoomies run in all directions.