knocking in 2005 international 7500 HT570

My 2005 International 7500 HT750 is knocking in high gear. It dont do it until you are in ninth or tenth gear and its only when your on the throttle. It sounds like somebody is hitting the engine with a hammer. Bang....bang....bang....bang. Had my usual mechanic check it out and he said he wasnt sure (he's usually pretty good about finding the problems). Any ideas or suggestions. Thanks for the help!!

 

I think we got it figured out. Gonna have to replace the back axle. Thanks for the help fellas !

 

Im sorry, he says its a crack in the front axle. Not the back

 

I originally said that problem was in back axle but what i meant was that it was in the front axle.