my boys do that exact same thing all of the time. As you said there is no growling, hissing or other signs of aggression at all. They did it constantly at first but now it's just once or twice a day, and occasionally at night. Toby always wins cuz he's a pretty stocky boy, but Havoc still starts stuff with him. I just figured boys will be boys and they just wanna wrestle and size each other up. They don't hurt each other and they don't seem angry before or afterward, so I don't worry about it, other than when they wanna wrestle on the bed when I'm trying to sleep.

much like yours, my boys don't really play with each other either, other than these impromptu wrestling matches.