Similar thing happened to me once. It sounded like I had a jet engine at WOT. Turned out one of the boost hoses was loose from the intercooler piping. Recheck that they've properly seated on the lip and that the clamps are tight.
"The 300ZX Turbo is a dance; it's a song; it's rolling, roaring automotive art. There is no color that doesn't suit it. There is no mood-lifting chemical substance it can't replace." - Automobile Magazine