We have some major ferry terminals here.. and the noise that transmits into the boat from a ferry running hard against its ramp while unloading can be impressive..
Quite possible you're hearing traffic....
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)