A couple suggestions:
1) If you jack the car on stands, and the sound is gone, it's something else. On stands and if the sound is there, you go underneath and with a prybar, place it near the mounts and pull up / down to see if the sound goes away.
2) Does this happen more in forward or reverse? With someone else in the car shifting from forward to reverse, watch the engine movement, if any. This mostly shows the upper mount is going, but you may see something underneath.
3) Can you get ahold of a stethescope? With the engine running and car jacked, crawl around underneath by the mounts and listen.

Hope this helps