Engine noise

Hello, all. I have been riding for about a year, and my bike is great. However, just last week the engine began to roar. Since it is a Ninja 500, it should whine, not roar. There is also a hollow knocking sound when I idle, and when I accelerate, there is a harsh growl. It sounds like the engine's about to backfire.

I am mechanically ignorant, so does anyone know what the problem could be, or a website I could go to to find out?

Thanks for the time! I appreciate it. Take care.