Is it possible

I don't know, but in my quests, i use the whistle flag and a whistle tile (so the user knows where to blow it). Using the whistle causes the secret to happen, and you can use Secret Flags (16 - 31) on unwalkable tiles to reveal the item.

Or you can use the Whistle-Stairs option under Data, Screen Data.

its possible to make a treasure chest appear with an item in it
this involves secret combos if u hold ctrl on that combo u can select a flag choose the flag 10 then choose flag 16,17,18,or any other secret flags
sorry if im not good at explanation

Edit: ops this second msg wasnt meant to be sent

How do that "Whistle->Stairs" work?

When you blow the whistle it will make the stairs secret combo appear where you put the yellow square on the screen. You have to make sure you set the stair tile in the secret combo menu and set it up as a tile warp.

Thanks. You were very helpful.

Just a note: You could make the item "appear" by having the whistle trigger a tile placed on a layer above the item itself, and then surround that tile with no-walk tiles that also vanish when you play the whistle. I've tried it out and it seems to work for me.