Will sometimes makes this sound. I'll do my best to spell it: "FurrrrrEEEEET" Luke and I had no idea what he was saying. Then one day we figured it out - we were taking a walk and Luke would whistle for Rosco and then immediately after that Will would make the sound. It's Will's interpretation of Luke calling Rosco by whistling. It's hilarious and has given us a lot of laughs over the months.