i have a 2006 g35 coupe with the rev up... i started my car one day and heard this sound coming from the engine.
Any ideas would be much appreciated of what it could be?

 

U either have a loose spark plug, or you have rod knock. Rod knock is bad news.

 

If you have a long screwdriver, you can put the blade end on various parts of the engines (be careful around the fans and belts) and put your ear on the handle and hear where the sound is the loudest. That will give you a better idea where the sound is coming from. The lower in the engine, usually the worst.

 

i went to my auto teacher in highschool and after changing my oil he recomended to check my belts to see if any were not woking or rubbing against anything... and after further inspection i found my ac belt torn up and frayed on the edges, so i decided to take it off and the sound was all gone.

 

Nice! That was the best case scenario possible lol.