This sounds like synaesthesia.

The part of your brain that interprets audio data is some how fused to the part of your brain that interprets visual as well. Resulting in the bleed through or ghost image of the flashes.

People have this to varying degrees of intensity and with other senses as well. Some people hear and then get a taste response for example.

That isn't to say it couldn't be something else.

Like a brain tumor

Synaesthesia was my first thought too. 

But........  do you find that if you cough or sneeze in the dark that you see flashes too? 

I'm thinking that maybe it's likely to be caused by some sort of internal pressure created by the cough, sneeze or the jolt of being startled by a sudden loud noise, an internal pressure or electrical impulse in the brain that excites neurons to produce a sensation of seeing a flashing light???

Or you shake so violently that the receptors in the eyes send a signal to the nerves. Receptors react to inadequate stimuli too if the stimuli are strong enough. And the brain receives the signal "the EYE has noticed something" and makes it into "there was something to be SEEN" even if it was in reality pressure or a very loud sound wave (bang) or something. Unfortunately, all receptors are constructed in such a way that it is difficult for the experimentator to give them a strong enough inadequate stimulus on purpose without doing any harm...

But really I think synaesthesia is more probable. I have it the other way round too: when I see a sudden flash I hear a crackling sound. In some cases there might have really been one, of course, but  in the case of lighnting I am well aware that its real sound is the thunder which comes later.

I sometimes find that if I get up and go to the loo in the night the light in the bathroom seems to pulse lighter with each heartbeat- I suspect because it is just that- my heartbeat pushing the blood to my eyes, causing them to slightly dilate with each pulse.  Could your flashing light experience be caused by a similar thing, the shock of the noise causing your heart to momentarily race and dilate your pupils ready for fight/flight?

Thats a polterguist mate!