I was also in an anechoic chamber a few times years ago. I remember once, I had been working on a robot for the FIRST competition for about 10 hours straight. I was exhausted, so I thought, 'I'll take a nap in the anechoic chamber'. It was probably the only quiet, dark place around.

I couldn't sleep, and lasted only about 20 minutes in the chamber. I didn't have hallucinations, but I did start feeling paranoid and the lack of sensation was (for lack of a better word) intense. I couldn't stand it. Perhaps I would have started hallucinating if I had stayed longer. Sensory deprivation is a truly bizarre experience.