Here is an opinion: https://www.merriam-webster.com/grammar/is-it-a-or-an

Hi Mark, I would also write 'an hallucination'. I know it is supposed to be 'a hallucination' because the word starts with the consonant 'h', but because it rolls off the tongue better, I would always tend to say 'an', just as I would also say 'an historical text' rather than 'a historical . . .'

A hallucination