Here's some additional general tips:
I tend to mark my location on the map every time I get pinged. I also draw a 1000 yard circle and keep it centered on my estimate of their current search area. It helps me visualize the whole action in my head, if you know what I mean.
If you just sank targets in shallow water, be careful not to maneuver your sub into the wreckage. I would certainly never admit to having done that one myself...
At some point, the destroyers are going to break off the search and rejoin the formation. As a general rule, after an attack, try not to evade in such a way as to put yourself between the destroyers and whatever they are escorting. If you happen to be in their path when they return to formation, they can find you again totally by accident.
Another big tip is to save often. The game is glitchy. I think the infamous CO2 bug has something to do with being detected when you really shouldn't be. I generally give the game the benifit of the doubt, but I have had occasions where I have been detected from a *long* ways out when there was no way that could have possibly happened legit.
Re damaged prop shafts making more noise, I honestly have no idea.