Is this an AOD noise?
 

My car: 91 5.0 LX Convertible w/ AOD.
I've been wondering about a noise that I suspect is coming from my tranny. It is hard to hear unless I am at low rpm (MAC after-cats). It is a low, dry, "chugging" sound that increases as rpm increases. I'm pretty sure it's normal because I have heard it in EVERY automatic Ford vehicle I have driven/rode in that precedes about 1994. Just wondering what the hell it is. Any ideas?