SEASON 4 SPOILERS I completely agree and at first I wasn't actually sure if it was John or my dog making the sound at the time. But after realising I think it is honestly the best choice of acting for what had just happened.
This is where Martin Freeman nails the character of Watson where some many other have missed the target. He IS a soldier. He IS a doctor. But he also types with one finger. He's incredible, but he's also just one of the boys.