Nobody really knows what causes it. i have a 21 Mach1 and I have no ticking. I change my own oil and use Motorcraft 5W-30 synthetic blend oil and filters. They say it doesn't cause any harm but it is annoying my friends F150 does it.
Usually when just draining the coolant from the radiator there will always be some left in the block. I would pressure check the the cooling system. I still bet you have a cracked cylinder or head.