My friend and I were cruising around today, when I decided to rev to redline. When my needle got to about 5500rpm, the sound tone and power of the car changed. The car got a lower pitch sound to it, and it felt like it was losing power at that point. It was like it was struggling to push through that 5500-5800rpm mark. Once I got by it, it reved fine. Can anyone help me and tell me what might be wrong?

Thanks