It's on the 300 SEL.
I shined a flashlight by the pump and I cannot see a water leak, yet it leaks profusely. I guess the only way to tell is to remove the tank. YOu are right though that I cannot imagine how a hole got in there when it was fine a couple of days ago. The only change was that I removed and reinstalled the water pump. I need to get a new seal(s).