It could be a few things. The first thing that comes to mind are the motor mounts. They have a rubber insert that deteriorates and sllows the engine to move father than it should and the mount can have a metal to metal contact in it that can make noise from the common vibrations a motor makes. The motor will lift and drop making the noise.
The way to check this is to inspect the motor mounts for the condition of the rubber that isolates the shock and vibrations.
Worn out suspension bushings can do the same thing, but are not as likely since you are sitting still.
Here is a link that covers all of the mlost likely posibilities including the motor mounts.
Wednesday, October 5th, 2011 AT 2:19 AM