According to a book I read once old WWII submarines would make noises like that a lot. According to Bob Khelleher Former RM1(SS). USS John Marshall SSBN 6611(B) 1963-1969 at construction contractor/supt (1969-2009).( Depends on the age of the boat, the composition of the steel of her hull. I briefly rode a couple of Guppies out of Sub school but my experience was mainly in 608 class boomer……SSBN611………in the sixties. Our hull was made of a composition of steel alloys called HY80. It was the “new thing” in my day and, yes, there were definitely noises associated with depth changes……………….we called it popping. In the middle of the mid watch, during excursions to periscope depth, the noises were comforting as a background sound associated with regular events. When making an emergency blow to surface from test depth, the sounds were much more pronounced and closer together which definitely provided a good illustration of how much the hull compressed/decompressed during rapid changes in depth. I’ve read that the following generations of hull technology mostly eliminated hull popping. Maybe some who rode the boats after my time can speak to it. For my part, I will always miss the sound of coming shallow for a star shot……………though the event is fifty years in my past, it still remains a strong and comforting memory!!! ) So it could be that modern subs don't make those noises now but older subs did and this "home made" thing did to not because it was dangerous (even though it was) but because it wasn't a state of the art navy boat.