prob not the reason mate,more likely just a bit of a yip as it has to make up a bit of ground to get back up on the bunny that's all,as said you will know a yapper when you hear it lol
so do you think it sounds like yapping. or just the dog really trying
never had a yapper myself,just would not do for the places i go that's all,but seen a mates dog give a little yip especially if the rabbit has turned quickly and the dog has lost a bit of ground on it,other than this the dog runs silent,i suppose you could describe it as a little yip of frustration maybe,a lad i run with years ago