The Morse Code

After Coors dies and the 4 sailors return to the sub there are some sounds like metal pipes hitting the sub on the outside. The sailors recognize that it is morse code and they hear B-A-C-K before the Brice shuts them up.

This was Captain Winters ghost saying "go back" because after that the sub DOES return on it's own to the spot of the hospital ship sinking/German ship attack.

Did anyone hear the full message? It seems like there are more letters, not just repeating. However, it is hard to tell with the loud dialogue masking the sounds.