I replaced all the internal parts of a american standard lexington toilet with handle on front of fill tank. All these parts are original american standard parts. Why is the filling part of the flush cycle so loud. I have turned the water down and turned the fill pipe inside the toilet to hit the side of the tank but did not help. This toilet is 15 years old

It is a common problem. You can actually purchase Fill Valve that fills water quietly. Other than that, try to lower house water pressure to 55 psi and keep angle stop fully open. Your flex fill connector should not by kinked. Also, make sure you don't have restricted flow by debris lodged in the fill valve...