[WernherVonTrapp]

I've heard that (sort of) heartbeat sound too. It's nothing. Don't try and chase after it. Ship screws have a distinctive cranking sound that is unmistakable and as previously mentioned, will activate the green indicator light. If you listen yourself, you can actually hear them as far away as 20nm, though I'm not sure how realistic that is. I've tracked down (and sunk) many targets using sonar. Early radar doesn't have quite the range and you can (fairly accurately) estimate the target distance and course with sonar.
To get the estimated distance and course, just use the ruler tool to draw a line from your sub to the tip of the sonar contact, then repeat the process in 10 minutes and draw an intersecting line through the tips of the two sonar lines. This only works, of course, if you have contacts on and can see the sonar contact lines.

The active sonar (ping) can be detected by some merchants and by warships so, be careful or you might stir up a bee's nest.