I think it's fan noise. After 3 years do fans get noisy?
Possibly. Dust gets in the fan motor and causes harshness and vibrations.
To make a computer quieter, you need to go through an iterative process of finding out the noisiest part of the computer and making it quieter, then repeat the process as necessary until you are satisfied. It is easy to do, but YOU need to do the work because your ears are near your computer, not ours.
The most likely culprit are the fans. Start by unplugging the SATA data cable from your boot drive so that you can turn off the machine at any time without having to properly shut down Windows. Then, unplug all fans except for the graphics card fans. Yes, even unplug the H80 push/pull fans. Don't worry, your CPU will be fine for the short amount of time it will idle with no load. After unplugging all those fans, turn on your system.
Did that cut out enough noise?
If so, then you have to do two things.
1) Reduce the number of fans. The Antec 1200 was meant to be a windtunnel of a gamer case. It has more airflow than even a 4.5GHz Sandy Bridge will need (not sure about your graphics card though).
2) Out of the remaining fans, replace with lower RPM units. Just replacing them may reduce bearing noise, but lower RPM will also reduce noise (as well as airflow).
If not, then the next possible culprits are the fans you can't unhook (PSU, GPU), the H80 pump and the HDDs. You can unplug power to the HDDs as well as unplug the H80 pump (basically turn on machine, listen for a minute, turn it off - it will be fine!). For graphics card, you'd need to remove the entire card. For PSU, you can momentarily stop the fan by using something to physically restrain it.