Cool air blowing from return vent

Hello all, I have problem with my down flow furnace. while heater is running cool air is blowing out of the return vent closest to furnace. The heater seems to working properly blowing out of all the registers . This the first time I have looked at the furnace since I bought the house a couple years ago. I had a technition do work on the furnace last year. I figured i would change the filter this year myself. I found every thing very dusty and the filter just to be a rolled piece of material jambed into the duct. it was hanging half out and not doing much of anything. I vacumed most of the dust out and found the proper support bracket for the filters laying next to the furnace. I have replaced the filters with the proper ones now. I am still getting the cool air blowing out of the return vent. Does anyone have a suggestion on why this is happening. Thanks Charlie

Weather King Downflow gas heater around 10 years old.

Sorry, folks I just went and stuck a piece of paper over the return vent and it sucked in. I could have sworn it was blowing out but it wasn't if I knew how to delete post I would Charlie