Hello!

 

Just bought 2018 Octavia RS. I have noticed that there's some strange rattle coming from drivers door when closing it. Sounds like side window are not properly closed. Took the car to dealer and they said that there's nothing wrong with the window regulator or motor. Still when i close the window, it falls couple millimeters. When trying to raise, motor is moving little ja window moves and falls. 
 

Also side mirror crackles little when closing door.
 

Any ideas what to do?

Well my window doesn't drop a 'couple of millimetres' when I close it, and it doesn't rattle either, so I'd respectfully suggest that the dealer is wrong and that there's a fault.

 

Either dismantle the door card and have a look for yourself (there are plenty of guides on YouTube) or if you're not brave enough, find another garage willing to investigate for you.

Chances are one of the screws holding the window runner have came loose and they are causing it.