Here's the info as written by GM for the 2011's. I was given a copy of it when I took my 2012 in for the same issue, but can't seem to find it now.
Here's a link to the service bulletin issue I was able to find on the web. And yes, acocrding to the document it is ONLY on the convertibles.
Text from article:
Exhaust Rattle Noise
By Blog Post on June 1, 2011 9:59 PM | No Comments
The 2011 Camaro Convertible SS equipped with the 6.2L engine (RPO L99) and the 6-speed automatic transmission (RPO MX0) may have a metallic or spring-type rattle noise that is usually heard at idle when parked next to a wall or when pulling into or out of a garage.
The exhaust system for the 2011 model year has been revised to reduce exhaust noises heard during Active Fuel Management (AFM) mode. This revision includes a spring-loaded valve placed in the exhaust system before the muffler. (Fig. 15)
t is characteristic of this spring-loaded valve to make some noise during engine operation or when tapping on the exhaust pipes. No parts should be replaced or repaired for this noise.
photo from article