In all accounts that I have read the Fahren descibe the sound as..."A hammer hitting the hull of the U-Boat". or "Pebbles hitting the hull" or "a steel cable/hawser being dragged along the U-Boat".
See this thread.
Link:
http://www.subsim.com/radioroom/showthread.php?t=142163
You can download my mod, above my siggy.
I have attempted to represent the sound of a high energy beam hitting a submerged U-Boat.
KUrtz