i have a 06 g35 coupe 6mt. Upon hard acceleration i can hear a very loud crack at about 5-6k rpms. Its one loud crack coming from the front end of the car. Crack usually occurs going from 1st to 2nd gear, once in a while it will occur going from 2nd to 3rd.

I will add a few more detailed descriptions in the morning as i heard this noise 2 days ago.

 

Could be a motor or transmission mount.