Is there any modern information about a 12-hour periodicity in sonar noise?
Some further googling seems to have shed some light: it appears that shipping traffic is highest at these hours, as demonstrated by Figure 3 in
http://cetus.ucsd.edu/Publications/Publications/McKennaIslandsSymp2009.pdf.
Why that is, I'm not sure. Also, this publication isn't about global shipping patterns, but only that in a certain region on the west coast of the USA - but it certainly goes some way to explaining the noise detected by sonar.
I know this was a very niche question, but hopefully this is helpful to someone who may stumble on my question in the future.