Why is my PS4 Slim fan so loud?

So I recently took apart and cleaned my friends PS4 Slim and after I put it back together and started playing CoD WWII, the fan was WAY too loud. I don’t know what the problem is, it was quiet before, but now it’s a roaring monster! I don’t know what to do. Any help is appreciated! Thanks!

Try replacing the thermal paste, and make sure the thermal pads are on the ram chips