A good question :hmm: I know it shouldn't be the GHG (group listening apparatus) because they were mounted either side of the bow on type IXC boats(eg. U-505)...see p13 of this article:
http://www.xs4all.nl/~aobauer/GHG1996.pdf However I don't see that on your picture
So I'm gonna gues it might well be a fifferent type of sonar housing or an 'active' system of sorts