My Macbook air has started making a low white noise.

Hi,

Try to run Apple Diagnostics if it is caused by hardware related issue(VFD001 to VFD007).

Use Apple Diagnostics to test your Mac - Apple Support

If it doesn't help or error was reported, you'd better make Genius Bar reservation if your local retail AppleStore is open or find opened local Apple Authorized Service Provider to be examined your MacBook Air there.

Genius Bar Reservation and Apple Support Options - Apple

Apple - Find Locations - Location Selector