You might need to replace the fan.... but since it's hard to tell from your description whether the "whirring noise" is loud/bad or normal, maybe the noise is correct for a hot computer. That the sound stops when you remove the AC power supply supply supports that its a fan because fans and screen brightness change when on battery power. Make sure you've cleaned out the inside of the computer with compressed air so that the insides can 'breathe'. Recognize also the sound being made could be from the heatsink fan, NOT the vent fan. So that might need to be replaced or a new thin coat of thermal paste needs to be applied on the heatsink/processor.

Just a note: Most REDUCE screen brightness when on battery to save power, but for some reason, you have it set to increase brightness when on battery. Your choice.

Hope this helps.

Grif