On that large a dose I would think just about any sensory hallucination was possible. Impossible to say what "caused it". Here hows this: some process in your nervous system that uses a frequency-based sampling loop to monitor some organ in the body or communicate to another sub-circuit in the brain, suddenly, due to that huge amount of LSD you took, was being inappropriately connected to and "read" as a sound signal by your auditory processing system.
As likely as any other explanation (and now that I think about it, a pretty damn good hypothesis).
Unless you'd prefer to think it was aliens trying to connect with you. If so, next time wear a metal helmet for your entire trip. And that will make a pretty picture for when they cart you away to the funny farm, haha, just joshing.