I am an active, serving sonar operator and can tell you that actual "pings" are not figments of anyone's imagination. I would imagine that most WWII era active sonars would have used the classic ping, but there were many different models of sonar developed and installed, and the allies were always looking for a bigger advantage, so you could have heard anything I suppose. But again, most would have been of the pinging type, probably using a continuous wave (single frequency) transmission, which sounds most like a tuning fork.
This page
http://www.jproc.ca/sari/ details the sonars fitted to a particular Tribal destroyer during the war. Parts 1-4 are what should interest you.