I was recently backing up a driveway in my `05 FX4...no weight in the bed. As soon as I shifted in park, their was a big bang...it did not happen again after that.....
Also, I had a new transmission installed in this truck at 51 K after purchasing it from a dealer in NY. In July I had it up on a lift, when 2 technicians were looking underneath from the transmission on top of the shield looking back, when I overheard one mechanic saying "it`s cracked"....and then when I asked what was cracked? they - ( typical behavior in a dealership) walked away from me...and not answering any questions.
What do you think??
Hankster

2005 FX4 5.4 auto Xtended cab :

 

I would take it somewhere else and find out exactly what is cracked........

 

x2!!!!!!!!!