I came across a really fantastic alternative to the Wallaroo encounter in the campaign book. It builds it up a lot more, and tells a much more well rounded story.
As suspected, the sound comes from an animal—a sooty albatross has found his way into the tank, and has broken his wing. The pathetic bird grows still upon being illuminated, staring up at the investigator with unblinking eyes. The bird’s feathers are ragged and covered by half-frozen whale oil, and it’s clear the creature has been living on the human remains.
Yikes. And that’s not all.