that sound happens every time you are starting too close to an object. sort of a contact damage sound effect error i think, but anyway just move your sub back away from the minefield or subnet and the sound will be gone. you also start game with hull damage if you hear the sound even though it wont be reported to you.

It's not just the start point i have found. I can start at PH, go to Truk, and I hear the noise at a seemingly random time near truk.

Sometimes where you'd expect, coming out of TC in render range, oter times no noise, then I try and sneak in and hear it later.

setting mine layers
 

I have had alot of success making mine layers so that they range from -20 to -60meters. To make it work right, if the density is only 50 and the radius that you make is only 1. then you will only have about 50 mines. Not effective if you want your mines at to be mixed between 20 to 300ft. I set my mines density to 300 at a 1 radius. Now you will have a better mine layout. I've made a map with this kind of mine settings. you can get it here, http://hosted.filefront.com/ddmobiltv/ , the file name is called Sneaking In. By the way, that sound that people sometimes hear, i've noticed that when, I have subnets in the game it does that sound, no matter how far I place the sub from nets or mines, but when i take out the subnets, it stops. huh?