My car is making a really weird noise after you start it and it is at high idle for about 20 seconds it makes a loud thump coming from the bottom of the car. Only does it once and only when the car hasn't ran in a few days. Temperature outside doesn't seem to effect it. It kind of sounds like someone hit the bottom of the car with a bat. Seems to be coming from the passenger side by the mid pipe. I got under the car and no scrapes or dents around any of the exhaust. If anybody has a clue what it could be please help me out .