Update your post to include full system hardware specs and OS. Is there, for example, a hard drive installed?
Finding the location/source of the sound is key.
You can use a 12" cardboard or paper tube to help locate the source of the sound. Put one end to your year and point the other towards the inside of the case. Carefully scan the insides (keep the tube out of the case) to find where the noise is loudest. Check fans and HDDs.
Also inspect all fans and wires. Ensure that there are not any bits and pieces of paper, styrofoam, etc. inside one of the fans and scrapping. Or even a wire touching somewhere and being scraped. Look for loose screws or anything that may be moving/vibrating.
Again the noise source is what must be found.