Dog looks like he is trying to cough up a hairball?

My 10 year old chow/terrier mix had a 3-5 minute bout of heaving/coughing up or trying to force something up and he looked kind of like a cat does and sounds like when they are trying to force up a hairball.I checked for a blockage in his throat but nothing was there and he didn't spit anything up,then he stopped and hasn't done it since.I thought that he was going to gasp for his last breath of air and keel over.He is in great health and appears to be himself ! Any ideas of what the heck happened? Any thoughts would be greatly appreciated...and yes he will be taken to the vet in 2 days...Thanks...