All, yesterday i just noticed something "special" about my '99 Bora 2.0 8v, APK engine. When i get to 4100 rpm (full or light throttle) the engine gets boomy, like a switch that doubles the engine/induction noise. Is that normal? WTF is it? Cheers Mark p.s. it does NOT have EGR, variable-length inlet manifold, variable valve timming or a secondary air injection pump.