We had some serious rain last night. Today, I was pulling out of a parking space, and had the radio off. I heard a weird "sloshing" type noise. When I was done backing and hit the brakes, I heard it again. Odd. No fluids in the back... Anyway, it's an '07.5 quad-cab Silverado 1500. I open the back door, and realized it had about a gallon of water inside. I swung it back and forth and it started draining out. I've got ~39k miles on it, so I assume this won't be a warranty fix. Take it to a dealer and ask? Or drill a drain hole or two? Or just take out the "trim type" plugs that are in the bottom? Ideas?