hi, it doesn't happen to me every day, too! Probably it takes place only in some circumstances.
I tried Narvik II mission, letting me pinging by DDs. The water is pretty shallow there and sometime I was able to hear it..
Edit:
OK! I was able to record it! now you can hear it in the link below, it's the second ping. I was in NarvikII mission, at 20 meters of depth under keel, and a DD had already started pinging me, the record starts just after getting the detection notice in events menù.
http://www.filefront.com/14797567/weird%20sonar.wma
So, I think it happens when a DD discovers you by its asdic, and it could be the echo.