I have a 2006 r53 with various mods ( 17%pulley, exhaust, cold air intake, egr reroute, ect.) wonderful car but recently at about 4500rpm I'm getting a loud squeal that almost sounds like the belts slipping and a yoyo in my boost. Apon further investigation it's not the belt. I've been looking into getting the Detroit tuned bypass valve hoping to fix the problem. Has anyone a had similar problem?

 

You may want to check the harmonic balancer. They tend to separate. Just run a search, there is lots of info on the topic.