Hi i have a 2009 Silverado Truck and my car is sitting at the GMC service since 3/15/2011. I noticed that knocking sound coming from inside of my truck. I remember the last time just recently i took my truck to goodyear for a oil change, then i notice the notice few days later.Like you, i thought it had something to do with the rain. GMC, is asking for us to say Ok go ahead and look inside of the engine to find out what is the problem. I was told if there is a defected problem it is covered in the powertrain warranty. If its not, we have to pay them $2600.00 for diagnostic alone. I have the same situation just like you, and still don't know what caused it or why is it making that noise. But i wont find out if i authorize GMC to look inside my engine. We have warranty til 2013 so i believe you should have warranty too.