Perhaps, gravel being moved in the surge? Doubtfull considering the depth and description of the noise. But you gotta start somewhere. I used to hear noise kinda like you mentioned when I worked on the Sum Fun. Down below in the morning filling the freshwater tank, I'd hear what sounded liked crackling ice or something. I was told it was the barnacles on the metal hull, but never quite believed it. Very loud, until the 3 deisels were fired up. Then it was hard to even think.

I realize barnacles aren't the issue, just a similar noise.

What about marine mammals?

Amish Ed
You Can't Catch it again if
it's Dead!