I developed a knock, sounded like it was coming from the head. Removed and inspected the head and all is good, while the head is off (engine still in car) I had a look at the pistons and they also looked good. I touched the top of no1 piston and it dropped down about 2 - 3mm?. It all turnes over smooth from the crank, everything goes up and down as it should but it seems this piston is loose causing my knock? Anyone else heard of this? what would be the cause, surely not just loose rod bolts! Will I be able to check if I get the sump off ?