I've got this exact same problem. Sometimes I get a loud whistling type sound in the higher RPMs. I've only got some basic bolt on mods, FMIC, split front/dump pipe, high flow cat, manual b/c at 10psi. Mine drops off to next to no boost past 5000rpm. My friends Dad had a look at it tonight, he's a nissan mechanic. We weren't able to solve the problem unfortunately. Got us pretty stumped.