Just for the fun of it, make sure you don't have a loose spark plug. Don't over tighten. The plugs are supposed to be gently snugged, like 1/8 turn after they contact the head. Sounds like there might possibly be a loose plug whistling.
To take this all the way you would break the plugs loose, shoot compressed air into the wells, remove the plugs, make sure the mating surfaces are clean, then reinstall and snug the plugs.