Furnace is making a whistling/humming sound
Yes I am starving for return air, My burner was short cycling and the HVAC company closed the hole near the furnace and added it down bottom near the fan. The burner was reaching high limit (too hot) from too small of a return duct. Former homeowner added a second return but wasn't helping. Since the one was added down bottom near the fan the short cycling has stopped. It was a cheap fix, but maybe not the best thing to do. The hole is about 14 x 16 roughly, maybe a tad larger. I wonder if it needs to be bigger??
I use the "mid grade" filters. A little better than the fiberglass but not like the expensive 3M ones