Chirp noise in the rear

[Tony S 3568]

I just bought a 2014 Sierra SLT a few weeks ago. As I'm getting to know the truck I've noticed a chirp noise in the rear when applying 10-15% throttle. It does not do it from a stop. Typically as going 25-45 mph, after coasting, then applying mild throttle I get a single chirp from the rear. If it were a u-joint you'd think it would be constant. Maybe leaf springs? Any thoughts? Anyone come across this? Thanks

[CamGTP]

Maybe it's the flapper valve in the exhaust behind the muffler. You could wire the thing open and take another drive, see if the noise is still there.

[Tony S 3568]

You nailed it. Followed a video I found to use a coat hanger and add a little tension to the spring. Thank you