screen issues on iMac AND macbook: what is it?

Recently, when I am on my macbook or Imac, i get a black screen for just a milisecond. I have absolutely no idea what it causes. But it is always happening when I am on Safari. Is this a problem from the new Mavericks or something else?

Secondly: sometimes, when I'm not on my iMac, i hear a noise comming from it. It sounds like something is active or something is starting up, when I go check what the noise is and I 'unsnooze' (I don't know what the correct term in English is) my screen, it gives a very weird black and white screen for half a second, right before it turns normal. The screen looks really weird: it are like blocks alternately black and white.

I'm kinda freeked out. I keep on thinking about hacking or malware because I have screen issues on both my computers. But I know the chances i got hacked are extremely small.

Besides the screens nothing else is weird and everything works perfectly.

What could these things be?