Oh my gosh, that "nibbling" is exactly what my boys used to do when they were playful. Gentle collie nibbles, up and down as if they were gnawing on a corn cob but not painful. Willie was especially nibbly. I used to sometimes play with them by running and they'd do their heel-nipping thing too. Willie used to use his paw to try and trip me!

When Cody was playing tug of war or something with me, Willie would just stand there and bark excitedly the whole time, that high pitched excited collie bark

Ah Par, your boys bring back such memories