Sophie Kate does that also, my Vet said it is actually a reverse sneeze and is common with Yorkies. It scared me the first time I heard it since she has a collapsed trachea also and I thought she was having a problem.
Here is the Wikipedia description of a reverse sneeze
Reverse sneezing - Wikipedia, the free encyclopedia
I would suggest you Google reverse sneeze, there are some videos that show dogs actually reverse sneezing.
Good luck.