RuneScape Classic Wiki
RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Magic golden feather
The Old School RuneScape Wiki also has an article on Magic golden feather.

The Magic golden feather is a quest item in The Holy Grail which points players towards the Sir Percival. By blowing on the feather it will tell you which direction to move in to find him.

After you have given Sir Percival the magic whistle, when you blow on the feather you will get the message "nothing interesting happens".