When I turn off my 2010 XF SC I hear a quiet buzzing noise that fades out after a while. Turns out it's the middle of the three internal lights in the roof just by the rear view mirror. The left and right are quiet but the one in the middle hums (it still works fine). Any suggestions - as simple as a new bulb, or something else?

 

Why don't you just replace it and see?
It's a standard T10 wedge globe, same as the dozen or so others on the XF, easy as to replace.
Or else swap it out for an LED globe, much nicer white light.