|When the processor is doing a lot of work it gets hotter, so the fan runs quicker to cool it more. When it is idle it generates less heat and the fan will slow down as it doesn't need to be cooled so agressively. When you stop the fan (not a good idea!) the processor overheats, even if it is idling, due to the lack of cooling. Then the fan will spin quicker until the processor has cooled again.|
The behaviour that you observe is quite normal.