Your AC whistles because the return grille is too small for the airflow, restricting air and creating noise. Undersized ducts and filters make it worse. Upgrading to a larger grille or duct fixes the problem.
Why is my return grill whistling when my air is on? It's whistling because your filter is too good. Now, it could be that. It could be your return side duct system is too small and your filter grill is too small. Bigger is better when it comes to your return. So you really, for all purposes in a residential application, you really can't have too much return or too big of return grill in your house. So the return grill is too small, therefore your return duct is too small, and therefore the air is whistling as it goes by because of a tight fit. So check your filter and probably your return duct is too small in that situation.
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