The "singing" can be a REALLY tricky one.....
You dont know what it is, dont have to be the mast, can just as well be the wing or the stabilizer, og the screws and joints.
Even wings made in the same mold, can be different, some sings others dont, if just a tad difference in how much resin and how it is finished.
Yours started silent, so I would look after imperfections in mast and wings, as small scratches can also make noise.
It can be the classic trailing edge of "something" being too round or too sharp, but it can also be the convex angle before the trailing edge.
Some in here gave the advice to try putting tape on the trailing edges one at a time, till the singing either stops or at least changes - then you are one step nearer the smoking gun / cause if it comes from there.
But as said, it is really tricky - if a typical cause on a specific brand, others might be able to tell you what it is, but because yours was silent at first, this is hardly possible