Check the bottom corners, one of them has a white rubber hose coming from the unit and may only be patruding out a little or you may need to look from close to the roof level to see it. Run a flexible wire, say #14 THHN into it about a foot to 18" and rotate it as you push it through. You should see debris and water coming out when its cleared. Yes, splashed rain can contribute to the full collector tray and leak through the duct, and so will a bad roof gasket that is between the roof and the unit. If you still have a rain leak after the tube is clear, then change the roof gasket. Good Luck!
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Check the bottom corners, one of them has a white rubber hose coming from the unit and may only be patruding out a little or you may need to look from close to the roof level to see it. Run a flexible wire, say #14 THHN into it about a foot to 18" and rotate it as you push it through. You should see debris and water coming out when its cleared. Yes, splashed rain can contribute to the full collector tray and leak through the duct, and so will a bad roof gasket that is between the roof and the unit. If you still have a rain leak after the tube is clear, then change the roof gasket. Good Luck!