Mine is doing that too. Originally I thought it was because I was using a texture pack that hadn't been upgraded to 1.4 compatibility, but it still does it without the texture pack. When I used the patcher and pressed "test minecraft" it read it as a "ran out memory" error. I've mostly noticed it happens when I was out looking for a wolf pack. Not sure if its just because its having to load the world faster when I'm just roaming around, or if it happens just as the wolf pack is being spawned. I know the rules say they only spawn within 24 blocks of you, which is the only reason I can think it might be the issue.
Its ok though, I usually use it as a my signal that I should go do something else instead. The pet wolf thing is kinda cool since I'm a dog person, but its kind of a gimick and gets kind of annoying after a while. Nice for hunting and gather of animal parts though.