Electrical outlet sings. Makes a varying, shrill sound. Why?

Cool! Singing electrical outlets ;-}

Sounds like you have a few holes in your exterior wall so air is getting through (in or out) so the holes in the outlets are acting like a whistle as the air goes though them and the outlet cover and/or outlet unit it's self. Get a smoke tester and holding it near the outlet you should see the air moving in or out when you hear the singing.

Inspect your external walls, repair any missing siding as well as use chalk and/or use insulation foam filling any gaps or holes you may find along the wall(s).

Internally patch any gaps along the floor boards and the wall and using spray insulation spray (low expansion) around the outlet box (between the plaster and outside of the box) to seal any voids. With the power off use some chalk to seal the holes inside the box when're the wires enter.

That should quiet the singing. In the spring you may want to get someone in to spray insulation in the external wall voids.