mine does it as well, cold start and 10 sec of knocking..
so I read other threads but can't understand how belts can make such a noise.. I'm gonna try throwing some water on them.
here you got some ideas mate
http://www.navara.asia/showthread.php?t=14855
I think this install looks the best (end of page)
http://www.navara.asia/showthread.php?t=3941&highlight=tweeters&page=3
boost controllers can only raise boost.. instead of installing a valve to lower the boost you should check why is it overboosting, have a look at your wastegate..
take some measures or make a cardboard box and try it out..
H x B x T 55,5 x 35 x 18,5 ( mit Bügel ) cm
Height x Width x Depth 55,5 x 35 x 18,5 cm (including the metal things)