My 99 makes a strange rattling sound when I am not moving but in D, shift to N and it goes away. Occaisionally also while in P. Tranny was rebuilt a couple months ago, and I do not recall this before the rebuild. This makes me think that it's in the tranny, but I don't know what to do next.

Any suggestions? Thanks.

Check the exhaust. Tap it with a rubber mallet. It is possible they disturbed a heat shield and it is now resting on the exhuast.

Sounds are one thing that you really just have to look for yourself. I can't hear it from here. :)