loud ping sound from shifting

Hi, I'm new to this (sorry if I'm doing this wrong) and am having a bit of a problem. I have a 1999 tacoma, 4 cyl., 2.4L, about 110,000 miles, and automatic. The problem is that sometime there is a loud ping sound when I shift into Reverse (and sometime drive). It doesn't happen every time (and pretty much stops when I try to look for the source, LoL) it seems to be coming from the underneath and near the front. I'm just praying its not the trannie (but is not lookin good). Any thoughts.
Thanks in advance.

 

Double post
http://www.tacomaworld.com/forum/4-cylinder/146017-loud-ping-sound-shifting.html