Car runs perfect. Sound goes away after long warmup. I thought there was a rattle noise near the radiator also. All started after negative weather. Always warmed car up before running and drive car 5 miles to work. Never run hard. New timing belt 5k miles ago and smooth idle.
I know this thread is a few months old, but if that sound comes back I found it to be the idler pulley on the accessory belt. When the bearing in my cheap plastic OEM went I thought the car was on it's last leg! It screeches and whines something horrendous. The aftermarket replacement is a metal version with a better quality bearing and sounds whisper quiet compared to the burnt out one being replaced.