2007 Vibe here. My first thought is that a hole has rusted through the muffler or a pipe, but I grabbed a flashlight and did some quick peeking and feeling around and didn't see anything.
Here's a quick video I took when I cold-started it: https://www.youtube.com/watch?v=ADuG3iOmEe0
From the sound of it, it seems like it's coming from somewhere on the right side, but again, I couldn't find anything obvious doing my quick peek.