Do you know if your unit has a fresh are intake on it? Usually in residential units the air is just sucked up from the 18" x 18" (in your case) return grill, heated or cooled and then sent out the supply vents.
Your original installer may have given you a fresh air intake as well which may be beneficial in diluting oders or if your house is one of those super efficient leak proof homes. If it is not in my opinion it is not needed so if you have one get up there and close it off with some cardboard or light sheet metal. There may even be a manual damper of sorts in it already that you can close.
If you dont have a fresh air intake, is your unit in your attic and not heated?
Could be when the heat is off the unit is cooling down in the attic and cold air is falling down your return duct.
If thats the case some type of automatic damper tied into your unit that opens and closes when the unit turns on and off may be the solution.
post some pics of what you got.
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1986 V-20
1986 Yamaha 150 HP
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