Okay guys, here's the situation. My nephew has a 2006 WRX. He has been getting a -4 to -11 feedback knock at 2500 RPM when accelerating. We have changed his ignition coils, removed all heat shields, spark plugs, cleaned the knock sensor, and replaced mass air flow sensor. We are at a loss of ideas. It only happens at 2500 RPM only when accelerating. It does not do it when coasting. Please help with any ideas.