Strange sound coming from graphics card

Hi.

I have a problem:
Yesterday when I was playing a game on my PC a strange sound started coming form my graphics card. I have never experienced this before.
I have an ATI Radeon HD 5870.

At first I thought it was one of the fans, so I removed the graphics card and cleaned the cooler and the fans as much as possible. I also cleaned the side fan of my chassi. The sound is still there.
I tried controlling the graphics cards fans manually and setting them to 100%. I could hear the fans speed up, but the annoying sound was gone.

Whenever I start a game, movie or other thing that makes my graphics card work the sound returns.
I filmed a video using my cell phone to illustrate the sound:

Strange sound - YouTube

Does anyone know what the sound is? Is it possible to fix it?
Can I be certain that the sound is coming from my graphics card?

Thanks in advance.

other suspects could be the CPU cooler.

try using a finger (or a pen) to momentarily stop (a second or so, not much more) the graphic card fan, then cpu fan and then chassis fan when you hear this noise. if it stops when the fan is stopped, then you found the device responsible of the noise.

fans do similar annoying sounds when getting older.