answered your own question, probably the booster. pull the hose and cap it. whistling goes away, found the problem. If that's not the problem, check any other vacuum hoses, for good connections, cracks, etc. Use carb cleaner or an unlit propane torch around intake and carb base, if idle picks up noticeably you have found the leak. my money is on the booster or cracked check valve where the vacum line connects to booster. good luck!