I can hear a loud fan sound when my MacBook Air is on.

Is it the fan??

That is the only moving part inside a MacBook Air. It might need to be cleaned of dust or other obstructive contaminants, which should only be performed by an authorized Apple repair shop, or your MBA might just need a simple reset. Try that first.

Power or other problems related to unexpected system sleep, shutdown, lights or fans call for an SMC reset. Read all the steps.

Before Resetting the SMC

Try each of the following steps in this order before you reset the SMC. Test the issue after completing each troubleshooting step to determine if the issue still occurs.

1. Press Command + Option + Escape to force quit any application that is not responding.
2. Put your Mac to sleep by choosing the Apple () menu from the upper-left menu bar and then choosing Sleep. Wake the computer after it has gone to sleep.
3. Restart your Mac by choosing the Apple () menu from the upper-left menu bar and then choosing Restart.
4. Shut down your Mac by choosing the Apple () menu from the upper-left menu bar and then choosing Shut Down.

Resetting the SMC on portables with a battery you should not remove on your own

1. Shut down the computer.
2. Plug in the MagSafe power adapter to a power source, connecting it to the Mac if it's not already connected.
3. On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time. The power adapter indicator light may cycle off / on once.
4. Release all the keys and the power button at the same time.
5. Press the power button to turn on the computer.

Apps can affect Mac performance, battery runtime, temperature, and fan activity - Apple Support