The cooling fan(s) (CPU and/or system) will speed up when more CPU activity is happening. Internet, multiple windows open, games, etc = more calculations/electrical signals being made = more heat = more cooling needed. No cooling = meltdown = buy a new CPU/motherboard, etc.
Now, you may want to check for lint and dust inside the system and air it out. Also, check the CPUs' heat sink for lint between the grooves; difficult to air cool if there's stuff in the way...similar to not removing the lint from the dryer filter, your clothes don't dry well. All of this does contribute to added heat generation/retention.