Engine Problems?!

My car seems to be making a metal scraping sound when the engine is running. It usually only occurs when I step on the gas pedal, whether it is in reverse or drive. Any ideas of what it could be? I cannot see anything visually wrong when I open the hood.

Does it make the sound at idle? Can you open the hood and get a rough idea of where it’s coming from? How about when going down the road?

Some more details would be helpful.

Yes it has been making the sound at idle as well as when I am driving, and when I open the hood, I cannot see anything that is rattling or scraping that might be producing the sound.