I have been having this intermittent exhaust rattle on my Corsa cat-back exhaust for quite sometime now and thought it was contributed to the weather change.
I tried on several occasions to re-torque the bolts to 45 Ft.Lb. of Torque to NO avail.
I then grabbed a rubber mallet gently hitting (one section) at a time and noticed it was coming from the very
TOP of the mid-section of the exhaust and found small pieces of rocks,gravel lodged between the exhaust causing this annoying rattle especially upon cold start.
I was fortunate enough to have an air compressor to get rid of those suckers once and for all. :)
Hope this helps someone out in the very near future! pics below,cheers. Z
(http://i.imgur.com/COzLDBUh.jpg)