I've had my '00 DTS for about a week and a half and I've noticed that when I stop for more than a few seconds (like at a red light, for example) there seems to be some kind of metallic popping noise coming from underneath somewhere.

Pop. Three or four second pause. Pop. Repeat.

The sound is similar to when an engine cools after you turn it off, but a little louder. And it sounds like it's coming from behind me, under the car somewhere - not from the engine compartment. I've tried putting it in Neutral or Park to see if this does anything, but it doesn't.

Anyone have any ideas?

Thanks in advance.
~Pete