What is low pitched vibrational sound when stopped in gear? It stops in N or P. on 2006 Toyota Tundra

When I put it in neutral or park the sound quits. It appears to be from transmission since it stops making vibrating sound. Any advice or solutions.

1 answer 2 comments
Possible motor or trans mont
Thanks for reply. Found the problem. Bolt on skid plate missing. Enough to cause vibration.