TuRbO-CaB-LaD

wen im drivin along an decide to put my foot down an go from 30 to 80mph...i get a loud knocking noise from under the bonnet, that lasts 3-8 mins then goes away, it is doing it alot now but only wen i boot it then slow down, wen i drive along wiv out turbo kickin in its normal, jus wen i cum on boost an slow down i get this tapping noise.......that makes my rs sound a right nail !!

any ideas??

(i had this problem before so i changed my head 4 a stage 3 head an piper cam an got it setup on rollers but it still does it an more often)

a shafted oil cap wudnt b 2 blame wud it??