I have a 2007 g35x sedan that makes a sound like this video when I start it up. After driving for a while it goes away, maybe bc the engine needs to be warmed?

 

Sounds like a belt slipping to me. May want to check the tensioner or the belt for wear while it's not moving

 

I don't think the car has a belt, it's a chain. It's nothing serious right?

 

The chain is inside the front cover and you can't see it. There are two serpentine belts, which I think jooky is referring to.

Look directly down you should see two black belts in the engine bay.

Yes, it can be serious if your main serpentine belt breaks you'll lose your power steering and alternator. The car will be hard to steer and then you'll be running off the battery and the car will eventually die.

 

See the thing moving in your video, THAT belt

 

It's not my video, just sounds the same as my car. But now the sound is gone, went away on its own