Are you sure it is not the air cleaner or something around the air horn on the carb? That is a common area for 'whistles' and squeaks. Try just taking the air cleaner off and put it back on and see if the whistle goes away for a while.
Used a stethoscope to narrow it down. It's the pcv, replace it and all goes quiet for awhile. It's the oddest thing to last only for a short period of time.
Hose is new and routed to rear a carb base. Noisy with air cleaner off.
