[meepmorp]

I dated someone who had sleep paralysis and hypnogogic hallucinations. She'd wake up breathing hard and panicked sometimes; said she was awake but locked into her body and heard these incredibly loud, terrifying noises.

Maybe get a sleep study.

[akomtu]

Sometimes, in that state, the "brain audio processor" gets disconnected from ears and connected to inner sounds - heartbeats and such. The texture to the sound is given by random thoughts, so if you happened to think about a waterfall, you'll keep hearing loud waterfall heartbeats. Even if you're well aware of what's happening, these sounds are still overwhelming. This is a common "OBE mode".