1993 Nissan NX muffler whistle (rev related)

When car is idling in park it makes a whistling noise from the exhaust and it also makes the idle rough when put into drive cars idle gos lower and noise get worse as im driving I can hear it and go with the idle unless u give it gas then it seems to go away with higher rpm id say 4 to 6 rpms when coming to a stop it makes the car almost die out u have to put it in nuetral to make it not sputter as bad the engine sounds fine but but the mufler whistles and it affects the acceleration