If any combination of shapes in your vision vaguely resembled a skull, you could hallucinate one when waking up from a dreamlike state.
The popping thing sounds similar to something I've experienced called exploding head syndrome. There are various manifestations of it but the most common is the sound of a loud explosion and the "feeling" of your head exploding.
When I experienced it, it was a bit different. I had a dream where I was in an alien spacecraft (and the aliens were all stereotypical greys with visible zippers on their backs like a cheap '50s sci-fi) and they put me in one of those "Matrix chairs". When they plugged the Matrix needle into the back of my head, I woke up with a flash of light and an electrical crackling and popping sound (and feeling!) inside my head, like an old tube TV. I learned it was entirely hallucinatory, but it felt like my brain experienced an actual electrical short from the inside.
Ah, that is most likely it. The "popping" noise definitely sounded electrical. It happened so fast I couldn't even react in any way other than an accelerated heartrate. ---
