Loud knocking sound on my Freightliner Colombia

I have a 2007 Freightliner Colombia ultrashift. Recently it started having a hard knock when it goes into 6th and then it happens again when it goes to 7th!!! A real loud bang noise. 8th- 10th gear it does not happen. But now while I just put it in reverse and started barely going back I hear the loud knock again!!!! Anyone ever have this issue??? Thanks so much for any help!

 

Well I told my mechanic about it he said I should be good to deliver the load. I get right to the location to sign my name in and bammm!! Clutch goes out and transmission will not go into gear. It started knocking going up the hills. I haul Frac sand so usually the roads have no shoulder. Thankfully I made it to the well site. Hopefully it's just the clutch and not the transmission aswell. I know right now it's a $800 tow bill to start with this nightmare.