I don't know what a collapsing trachea sounds like either - but I do know a reverse sneeze bc he has had that since he was born. It's much 'wetter' - if that makes sense. Almost like he is gurgling water a little - this new sound is a dry honking - i actually have a few videos of him doing the coughing that has just started a few days ago
http://www.youtube.com/profile?user=Wonka1970
Sometimes it starts when he is just sitting still (ie no activity/excitement or drinking water- like with the reverse sneezing).
I am sending this video to the vet so hopefully will figure it all out on Wed.