What would cause panting and frog-in-throat sound for an older dog?

My 13 year old Blue Heeler has been panting more frequently lately and there's a rough sound to it like he has a frog-in-his-throat. I've listened to his lungs and they sound clear. This is a dog who's been extraordinarily healthy and active throughout his life. He hasn't shown any other signs of problems (he's not lethargic or restless and he doesn't seem to be in any pain).

He always gets his annual shots, he's on monthly HW preventative, and he has his teeth professionally cleaned regularly. Again, he is a very healthy dog in a very well pet-educated family. He's neutered. The only major problem he's ever had was a torn ACL that we had fixed a few years ago. They always say that Blue Heelers will get injured before they ever get sick! Oh, he's also on monthly flea/tick prevention drops.

His last vet check up - probably about 6 months ago - went beautifully. The doctor raved about his heart and lungs being in good shape and that "he only looks like he's 4 - not 13".