It probably doesn't happen, ever, when you first start driving?
If it onlyhappens after the car warms up, the exhaust is getting longer and one of the bent rods, that hold the muffler in the car's rubber-mounted hanger is rubbing. That's what happened to me, anyway.
Super easy to fix.
Take a big pair of channel locks and bend the hanger away from the place that rubs, or loosen the pipes and shove them in a little farther. Whichever makes you happier.
2006 Black, GT, spoiler delete, five speed, leather, custom welded straight pipes with resonators, sequential turn signals, Hurst Pro Performance shifter. FRPP 4.10s with Motive bearing kit, Spydershaft one piece aluminum driveshaft, w/ Spicer 1330 U-joints, Steeda UCA and LCAs, chrome moly panhard rod. bye bye wheel hop, hello traction! SCT XCalII, BBK CAI, ZEX 100 shot.
I've written a book or two. Google David Buschhorn to laugh at my scribblings.