Cold Air Return Filter

I was feeling energetic spring cleaning last weekend and removed the cold air grate from the wall and gave it a good scrubbing in the tub. Before I replaced it, I looked in and there were massive amounts of hair and dirt on that little "floor" behind the grate -- I guess that's the top of the furnace? I started removing handfuls of hairballs and tumbleweeds laying on top, and kept going until I got down to the metal. My house was built in 1956 and I've never removed the grate and took out all that hair before. We just had a cold snap the past two days and I turned the furnace on. I can hear the air freely going into that grate now (where I never could before), b/c all that hair and debris isn't buffering the airflow noise any longer. Started thinking -- was part of that hair and mess that I picked up also possibly a rotted filter laying on top? Like I said, everything I brought up was in black clumps so I'm wondering if there wasn't a dry rotted filter in there too? Did I do something wrong or is that space supposed to be free and clear? I hope I didn't just provide a direct route for hair and debris to go straight down to the furnace b/c there's nothing there to trap it now. Help please. Thanks.
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