I just deleted my carbon canister and found the vapor drain hose to be clogged. This was causing excess pressure at the gas cap and the whistle you're hearing is probably from the vapors trying to escape. I would sugest pulling the vapor line and trying to blow air through it from tank side towards the CC. If you cant then you have a clogged line/carbon canister or a broken vapor relief pressure valve depending on where you pull the hose from.