Aeration is caused by an overfull condition or the pump sucking air...in some rare cases an internal leak will stir up the fluid enough to cause aeration. The problem is, aeration will cause whining, buzzing, etc. and so will an internal leak.

Whining/buzzing in the absence of aeration can be caused by a rapidly cycling pressure regulator valve, this has been a TF problem since day one...more of a nuisance than a real problem. A fluid/filter change might help.