You might try using a section of heater hose between the carb and your ear to locate where the whistle is coming from. Had a friend that complained he was getting a whistle coming from a carb I had rebuilt for him. Found the problem with three feet of heater hose. Turned out to be a sucked intake gasket. Same trick works for diagnosing water pump and alternator noises too.
I had that exact carb make a whistle like you described. It was a loose mounting/throttle plate, not seated flat on the manifold.
