The whistling sound is because of a restriction in the water supply. It works just like you puckering your lips to whistle.
If the whistle is at the moment of flushing there may be a bowl cleaning chunck lodged in the inlet. I do not approve of bowl cleaners/deodorizers that have to be hung in the tank, they give this sort of problem all too often.
If the whistle is while the tank is filling there may be a supply line of copper that was cut and not reamed; the same problem, it makes like a pucker.
Another thing you may need to check is the cut-off valve at the wall. It has a rubber washer in it that may have broken and is lodged in the supply line.
It may also be a problem in the ball-cock valve which you may need to change out.
Please let us know how it comes out.