In world 1-3, there's a secret warp whistle you can find by crouching on a white block, which makes you drop into the background after a while. If you get to the goal behind the black wall, you enter a toad house with a warp whistle.
In world 1-1, there's a white block like that as well, and it also allows you to drop into the background, however it's at the start of the level, far from the goal. Is it possible somehow to reach the goal of that level before the invisible timer (for when Mario goes back into the foreground) runs out? And if yes, what do you find there? Another warp whistle?
Best Answer
As you mentioned, the only white block in 1-1 is too far from the end of the level to get behind it with the crouching trick. The effect of going behind objects wears off by the time you can reach the goal. No guides or secret lists ever mention a magic whistle (or any other item) in 1-1 that can be obtained in such a way, even with glitches or hacks.
This link appears to show someone hacking the game to place a block near the end of 1-1 to find out if anything is hidden behind. I cannot guarantee this reflects an otherwise-unaltered game however.