Sounds like a simple vibration from a bolt or nut that occurs at a certain engine RPM. My wife's car does it all the time, and the SVO does it, my old Mach used to do it... Just gotta dig around until you can find the source. I could be way off, but it's the free-est thing to try that may get rid of the noise.

Does it make the noise when the engine is still in it's "warming up" phase, like early morning at the first stop you make, or is it after you have been driving a while and the engine is warmed and set at it's normal RPM while in gear at a stop?