Whistling sound from UPVC window area during strong winds ?

One window in our house seems to whistle during strong winds. I can sort of feel a general draft, but can't locate it. I can't see or feel where it is vibrating to cause the whistle.

I'm not absolutely sure it is the window frame. I can't tell if something outside such as the guttering could be causing this.

Suggestions as to what may cause a whistle would be appreciated.