We've had this question before, but it's been awhile. U-boatmen have said that you could hear it, but it sounded more like gravel being thrown against the hull. Others said it sounded like the hull was being hit with a hammer.
Here are a couple of threads from awhile ago. The first one has an answer from
you. The second one has some good links.
http://www.subsim.com/radioroom/show...ic+sonar+pings
http://www.subsim.com/radioroom/show...ic+sonar+pings