What type of screwdriver is required to open the fan?

Dell usually uses philips PH0 or PH00 screws. Make sure you double check that the screws around your CPU and or GPU heatsink are torqued down. It is not uncommon to forget to tighten one down. Also, try just using a compressed air can or compressed air to blow the dust from the fans / heatsinks. If you don’t have access to that, very lightly dab water on a q-tip and try to remove all the dust that you are able to reach. Do this only to fan and heatsink components and not circuit board components. Good luck!