They have no proof of what it was exactly, which is probably why there's nothing definitive on Wiki. I've just read articles here and there that have mutual postulations.
There's actually quite a few unknown sounds the NOAA have recorded over the years. I think "Julia" is far more distinct and disturbing than the Bloop, because it sounds the most organic, enough to earn it the personified title. You can hear it
here.
The other interesting thing is the most famous of these recordings almost all took place within the span of a few months in 1997, with a few exceptions. There haven't been any significant recordings since, despite the improvements of hydrophone technology.