had same problem with mine, after driven on boost the top end wud make a knocking noise then eventually going away after 5-15 mins,

so i changed my standard head an cam for a stage 3 big valve head an piper cam an vernier pulley, an it still made the knocking noise after driven on boost, wen i got it on a rollin road the oil cap blew off,
so i changed my oil cap for another 1, drove my rs hard for about 30 mins an it didnt knock once!

so i wud change the oil cap as this was my problem an is pennies 2 change ova, not sayin it is ur problem but worth the try