Only Simon can find Knockers. The first one is in a cave at the Quarry. Once you extinguish the lava with your Splattergun the Knocker will come out to give two Boomstones. The other Knocker is in the end of the Cellar Tunnels. You will find a note that says that the Knocker will only come out if you "ring" the crystal with a stone. Look behind you to see the crystal. Throw a Boomstone at it and the Knocker will come out.