Try the timing cover trick. I had to do that to my Zetec when I got the head overhauled. If that doesn't work, it could be detonation. If you had the head milled, that could raise your compression ratio. You could try the next higher octane rating of gas, drive it around for a while, and see if it goes away. If it still knocks, go to the highest octane rating and should stop it if is compression related.