The legend of chambers beneath the sphinx is quite old. The most recent claims were by Edgare Calyce. Egyptologists have never found evidence of any hidden chambers. They have even taken sub-sonic readings. The sphinx is actualy carved from the bedrock of the surrounding land, not built up out of bricks. For there to be a 'hidden chamber' it would also have to be carved out of the bedrock, then carefuly hidden by the placement of blocks to hide the opening.