Knocked Back On Repair

TWO years and 20,000km after I bought my 2004 Mazda 2 Neo new, it developed an engine knock, which sounded like a very loud tappet noise. After checking it the dealer said it was piston slap, but not bad enough to do anything about. I have now done 52,000km and the noise is now very bad, lasting for 60 to 90 seconds until the car warms up. My five-year warranty ran out in April and being an old guy I worry if I am going to have trouble later on. What can I do about it?