Once again JJ is right, air bubbles will make it growl. What bothers me is it is after working for a while, you may have a hairline crack letting air in or a bad filter, either way I would drain the system and put in the correct trans hydro or hydro fluid in. For $50 or so in fluid I would not keep running it this way.
New filter and fluid last week ----
Hairline crack :smiley_aafz: I hope not