The reason is that when the temperature of CPU rises after working for a period of time, the cooling fan will start, and it may cause noise if it is dirty or broken. Hibernation is equivalent to rest. When the CPU is not working, the temperature will be low, the fan will stop, and it will be silent.
The simplest dust removal is that you use a household vacuum cleaner and suck it at the fan outlet of your notebook for a while, which may improve a little. It's best to send it for maintenance inspection.