my MBP makes tick sound

I have a 13 inch mid 2012 MBP. I keep on hearing tick sound from my laptop and it happens at random or in a consecutive. I got this annoying problem just a day or two ago. So far I've tried, opening the case up and clean out any dust and retighten the screws although, this MBP is missing 1 extra screws because it was handed down to me from my sister. Another thing about it, it makes tick sound with and without movement.