I get the same thing. Im just about asleep and suddenly theres this clicking and sometimes a loud roar with a feeling like someone had hit me with an electric shock.

There is a medical word for it but I can't remember what right now. I was told its not something that will kill me its something about how the body and the mind sleeps in cycles. first theres fully awake then ideally the body and mind relax at the same time achieving deep relaxation and then there is sleep and the sleep cycles of REM and so on. But some cases the person hits deep relaxation then they get the "falling" sensation and experiences a type of panic where they jerk awake causing the body to jump out of deep relaxation which pumps blood through the heart and arteries and veins super fast. the sound is the blood rushing through ans the slight pain is the shock of so much blood going so fast kind of like eating ice cream too fast and getting a brain freeze. and this usually happens when the person is over tired. I was told to do something relaxing before bed and go to bed every night at the same time so that I get enough sleep.