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You could have a leak in the drain pan from a small crack use a regular silicon caulk or "pan patch". Please make sure pan is dry and unit off for curing. Additionally please make sure drain has only a single trap. You can confirm by making sure the line is draining outside with the unit running. Lastly please confirm there is no restriction in the supply air that would cause the unit to be starved for air and create pressure inside the unit. Look for things such as plugged filter debris on the coil furniture in front of return air grill or collapsed ductwork.