I had a diagnostic run, and they said that my head gaskets are blown (but I have not oil leak nor water in the oil), cylinders 1,3,&5 are misfiring and the others are intermittent, the water pump was going bad, and there is a leak in the block (possibly just because the water pump is bad). Water pump did go, and I had it towed home. We replaced the water pump, and then started to flush the radiator and put stop leak in it. While running the car for about 5 minutes after putting in the flush solution, we revved the car just a little to help get the flush moving through the system and all of a sudden the engine started making a horrible loud knocking sound. We of course immediately shut it off, but now I am scared to turn it back on. Is the van done for?