It's because the thermal pads are not good anymore. I strongly advise to just replace them (it's cheap), cutting the fan wire will only delay the issue because the thermal pads will get to a point where they don't do their job at all.. The thermal pads conduct the heat from the chips to the heat sink and prevent them from overheating, however they are not eternal, they last few years before you need to replace them. you can use thermal paste, it's better but because some chips are small, it can be messy.