i have a 1986 rx-7 and the other day while driving the car made this grinding noise. the car as a auto trans. what could it be?

 

Check fluid.

But other than that, I can't help you, I know zilch about autos.

 

when was the last time the tranny fluid was changed?

and the tranny filter?

Chances are they are dirty. They should be changed/cleaned every 30K miles or 3 years what ever comes first.

 

Most of the time autos start making noise is because they need the basic maintenance done. Change the filter and fluid and hopefully it will stop no significant damage done. If not the next stop is a tranny shop.