Computer Loud and I can't figure out why?

Ok my computer has been loud for like a month or two now. So I just cleaned it out like 3 or 4 days ago.. and I found a piece of plastic from my spray can i use to clean out the computer..And I found LOADS of Dust around my front fan. So I cleaned that pretty good. For the next 3 or 4 days my computer has been pretty quiet. Just a low noise. Now yesterday and today its been doing this crap where it starts off sort of quiet and then builds up in loudness and it like goes up and down like unsteadily loud noise, Its really annoying, and now its sort of quiet. What is the problem? I know its the front fan cause I can only hear it from the front. and when I tap the front right side of my computer it effects the fan's sound.. sometimes doing that makes it sound quiet again. What could be the problem?