I have the same problem. Its a little flapper valve located on the far right bottom corner of the heater box. Its some type of fresh air or recirculation flapper. If you will notice you can move your controls from different settings and it will make it flap doing that. I have put up with it for about 2 years and wish I knew the fix. Let me know if you do. Shayne7s38695.5911226852
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