What you're hearing is probably the driven plates being pushed by the hadraulic forces produced by the driving plates as they spin through the oil in the case, unless you've been really beating on it, in which case the basket and hub might be worn where the plates engage them and you're hearing the plates rattle in the basket. Like pauly said, if it's not slipping just ride it. When it does start slipping or if you find that it's hard to get into gear or hard to find neutral, you probably need service.