Okay, I'm not really expecting that anyone will know what this is from my simple description. Twice now in the last 10 days I've heard this same strange noise immediately upon startup. I turn the key, the car starts normally, and within a second or so I hear this noise. It sounds kind of like if you had a 1'x1'x1' cardboard box and held it waist high, then dropped it onto a concrete floor. The first time my wife was on the other side of the garage wall and heard it inside the house and thought something had dropped in the garage. I've checked under the hood both times it happened and didn't see anything out of the ordinary. Car starts, runs, and drives normally. Not a broken belt, doesn't sound like a bearing or something was frozen up and broke loose. The first time it happened we later found a box turtle in the closed up garage, so I'll check on that again later today too!!