My 2003 Aerio did the same thing. Rattles at slow speeds and when I stop at lights. Took it in to have it checked and they thought it was just the heat shield rattling. We took it off and drove some more. A lot less noise but still a rattle. Finally realized that it makes noise when the fan is turned on. The mechanic said it is the AC compressor. He suggested replacing it before it threw the belt which controls the water pump. He said if it does, the car would stop and I would get stranded, engine ruined, etc. Possibly the former owner never used the AC enough? He says it retails for $1500 but can get it for $900. Checked online and found them (same exact part number) for $314. I am trying not to think negative thoughts here about the price difference! Husband says to get a second opinion before shelling out $1000.