Hard starcoin on 2-4 i think or maybe 2-5

Answer from: Sel
Go down the yellow pipe and u will see a brick, get rid of it then walk to the other end and walk back without jumping there is an under ground bit.

Answer from: Golden Mario
Level 2-4 is hiding 3 star coins.the first one is right after starting the level where you see the box of random items. under it there will be a koopa troopa. grab it and carry it a to a small gap that is closed by a brick. throw the koopa al the block and destroy it. if you are super mario you will have to crouch into the gap to get it.the next one is after the check point. go staight dont ump onto the platform look up and you will see a pockey and a yellow warp pipe. get in it and you will see a pair of yellow platforms and 2 pipes spawning goombas you will see a block destroy it and die! just kitting go down and a platform will go down run to to the right and get yor pirze-star coin. the final one is not far after i. go upp all of the platforms. you will see a gap with a lot of coins. dont go in it jump over it there will be a gap go down and you will find your prize