Regarding the servcing of transmisson fluid, do I need to flush it after I've replaced the transmisson fliter?

And one other question, its some sort of a grinding noise that I've noticed when I lift my foot off the brakes slowly. Whenever I drive off after a red light I lift my foot off the brakes slowly and I hear loud metal grinding noise. I've checked my brake pads there is still plenty left and I've also checked my rotars they seem to be fine.