1995 Windstar Loud Buzzing from trans

1995 Windstar, 3.8L. 160k. I have been getting a loud buzzing from the trans. It sounds like it is from the bellhousing area. It is most pronounced when the vehicle is cold, regardless of outside temp. The noise ceases when put into a forward gear and does not return while being driven. If the vehicle is warmed up, parked for a while and then restarted, the noise returns but it is not as loud. It also goes away after the vehicle is put into a forward gear. I have changed the fluid as recommended on schedule. This trans also had a new pump installed under warranty by Ford at 75k. Any Ideas?