It's just the wind, that's the excuse Clara tells herself since leaving the Doctor.
That's her answer for every sound, however near or far it is; it's just the wind.
It's the reason for her reaction to every slamming car door, footsteps or creaking of stairs; she just blames the wind.
To her it's better than admitting the truth that she still checks to see if he's there but knows the Doctor's not coming back, that the TARDIS isn't out there, that the man she loved who became her best friend is somewhere in the universe traveling the stars without her.
It masks the pain and sadness she feels when she remembers he's gone.
Blaming the wind is easier to accept then telling herself that she thought she had heard his distinct Scottish brogue on the other side of her door, again.
Still Clara reacts to sounds in hopes that the Doctor came back or the TARDIS is outside. At night Clara hears something and she casually looks out her window but there's nothing there and she'll tell herself it was just the wind.
Loneliness can play cruel tricks on the heart when you're missing a friend and despite her efforts the realization is still there and the tears fall while her heart breaks because it'll always be just the wind.