Whistling on acceleration

So, my 2011 Dodge Grand Caravan has always had a slight whistle on acceleration. When my flex pipe broke it lost the whistle and all of its power. Recently I put a new exhaust on it, and it brought all of its power back, and has made this whistle significantly louder than before. I don’t know exactly what it is that is doing it, but I just wanted to know if anyone knows, since I’m curious and my research has gotten me nowhere.