I've had this many times before too.

>if I’m about to fall asleep and I happen to hear a loud noise nearby,

https://vester.si/bodily-oddities/oddities/hypnagogic-halluc...

>I also see a flash of light with my closed eyelids.

You might want to talk to a optician about that. It can be a sign of issues with your retina.

I see flashes in a dark room when I wake up, because I have PVDs (posterior vitreous detachment) in both eyes.

Edit: I just re-read your post. If they both happen together, I think it is something different.

Could it be that both noise _and_ flash are in your head, and like me you have Exploding Head Syndrome? https://vester.si/bodily-oddities/oddities/exploding-head-sy...

Weird. A spike in blood pressure maybe? Can you do reproduce it consistently on purpose with an alarm or by holding something as you fall asleep that will make a loud noise when it falls?

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