My wire fox terrier is retching but not vomiting and this has been going on for a few minutes. Why?

He was sick last night just the once but has been fine since. Then today he started to retch so I put him outside but no vomit. I had given him two Markies and 2 dog chocolate drops just 5 minutes before and he ate them fine. That is about the usual amount of his mid day snack. (he gets 3 walks a day so he isn't fat). He likes his Markies. He has been retching for 5 minutes but I can't see the problem. I looked in his throat and found nothing obvious. Help.