Rattling Noise Only at low rpm when transmission is in gear

[dennis]

I started having this strange sound a few days ago, or at least that is the first time I noticed it. It happens when I first go into gear at low rpm. It sounds like a heat shield rattling and is not related to a cold or hot engine. It's easy to hear when I am driving alongside a car, fence, etc and the sound reflects back. I checked everything I could get to so far underneath the car and nothing is loose, but it only does it when the engine is at low rpms in gear, nothing when you gently rev engine in neutral. Does anyone have any ideas? Driving, the engine seems to have plenty of power and idles fine.