Rattle under car at low, low RPM (in gear, but stopped)

[cleveland_saab]

I have a 2005 with automatic transmission (A/T is a sin, I know). Sometimes, while in drive or reverse BUT STOPPED (i.e. my foot is fully on the brake, like at a light or when you first put the car in gear), there is a low rattle that is emitted from under the car. Shifting into neutral brings the RPMs up just enough to immediately halt the rattle, and so does giving it a little gas.

Since I can only reproduce the sound when the car is in gear and running, I can't exactly jack it up and take a look. I'm sure that if I looked long enough, I could find something loose under there (heat shield maybe?). But given the recent drop in temperatures, I'd like to minimize my time lying on the ground outside. So if any of you have encountered and defeated this rattle on your own vehicles (or have good suggestions), please let me know. Knowing this community, SOMEONE likely has (and probably has pictures to boot!). Thanks in advance!

[Sno93]

Check the heat shield covering your muffler.

[BKNJ]

I'll second the heat shield on the muffler, I was able to get the sound to reproduce when closing the trunk. There is a shield above the muffler, and it would only vibrate at key spots in the RPM range. Sympathetic harmonics.

Bryan

[Sno93]

Yeah, mine got to the point where it would make a very loud rattle anytime I drove between 15-25mph. Very annoying.

[cleveland_saab]

Thanks Sno and Bryan -- looks like I'll have to crawl under there tomorrow night and feel/look around the muffler.