Noise From Engine

I have a 98 honda civic LX (157k miles) and I have had a noise coming from the engine. It only happens when I press the gas for the first time or when I am sitting at a stoplight. The sound tends to go away after pressing the gas for a couple of seconds when accelerating. It sounds like a knocking sound that speeds up or slows down when I press the gas. Can anyone shed some light on what this might be?

I'm a newbie at working on cars and trying to find out as much as I can.