I have a 2005 STS with about 22,500 miles. When it sits for a week or so it makes a loud noise when started. It only lasts for a second or so and it's different from any engine noise I am familiar with. I will try to remember to open the hood next time and see if I can localize it. It goes away instantly and does not come back unless the car sits for another week or so. I wonder if it is just the valve train making noise afer the oil drains back. Anybody else have this noise? Normal? I could never take it to the dealer for this unless I wanted to do without the car for at least a week. Using M1 5W-30 oil all it's life.