Clunk sound @ WOT...

My 02 GT 5-speed has 19K miles, and I think my Tranny could be messed up. When I run WOT and shift from 2nd to 3rd, my shifter will have a little pop or clunk to it. I took it to the local ford dealer, and he said I might not be pushing the clutch all the way in. :bs: It only does it @ WOT??? I'm puzzled and will seek and second opinion. Anyone have any ideas?
 
Is it your shifter itself or does it sound like its coming from underneath the car? If it's coming from underneath the car it's most likely normal, they say our cars have some drivetrain slack so sometimes when you're getting on it and you go to shift it'll make a little thump.