Narrated Abu Burda:
My father said, “I came to the Prophet and saw him carrying a Siwak in his hand and cleansing his teeth, saying, ‘U’ U’,” as if he was retching while the Siwak was in his mouth.”
Prophet (SAW) was very particular about his cleanliness and hygiene. He took special care of his mouth so it won’t reek. Miswak/Siwak was his best friend. He used to clean his teeth with every wudu and every chance he’d get. The implication here is that he used to clean his tongue with the Siwak as well, because germs tend to accumulate on it and cause sour mouth to stink. So he’d clean hi tongue as much as he could, going as far back as possible. So much so that the sounds u’ u’ used to come out (of the like when you’re about to vomit).
Try doing that now! :)