Beavers are known for their danger signal which the beaver makes when the beaver is startled or frightened. A swimming beaver will rapidly dive while forcefully slapping the water with its broad tail. This means that the beaver creates a loud slapping noise, which can be heard over large distances above and below water. This beaver warning noise serves as a warning to beavers in the area. Once a beaver has made this danger signal, nearby beavers dive and may not come back up for some time.
Animal Planet - Beaver