Probably not dust or old paste. Probably the original tight seating isn't as tight, maybe a push pin came loose or your fan's RPMs slowed down.
I think this is it, both my fan's RPMs might have slowed down and maybe even a push pin because I actually smeared some of the thermal paste off when I initially mounted the CPU. Thanks, any more tips guys? Maybe it's just my room temperature because I usually leave my room's door and window closed a lot of the time.