Don't know the answer but according to the service manual, taken from this webpage, the left fan is used to cool the GPU and the right fan is for the CPU (see p.39 of the manual).
Perhaps the GPU needs to have the thermal paste refreshed or there may be a problem with the heat sink assembly over the GPU.
It could also be a problem with the fan itself. Not sure if the fan has an internal thermistor which senses the temperature which is relayed back to the BIOS to alter the speed or if it is controlled by a temp sensor in the GPU but check the fan’s wiring connector anyway for anything amiss.
According to this link you can check GPU usage in Task Manager, to check if it is high GPU usage that is causing the fan to run at high speed. (my PC is too old as this option is not there.)