Exhaust noise

Hi i have a 1967 Dart GT 383 with manifolds 2 1/2 dual exhaust front to rear flowmaster mufflers. At about 1600 rpm i start to get a high pitch sound and it increases to about 2400 rpm. To a point you cant hear the radio it drop of almost completely at 2500rpm. I have done some research and a lot of the suggestions are to install a cross flow pipe.
Anyone else had this problem and tried this as a solution?