Derek Cole explains why return grilles whistle when the AC runs. A clogged filter, undersized grille, or small return duct causes the noise. Fix it by checking your filter or enlarging the grille and ductwork.
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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