So five months after (I didn't realise it was so long) I first suspected there was something wrong with my power supply, I finally got around to installing my new one.
The scary thing is, that when I tested my Thermaltake after I had removed it. I found that it is in perfect working order and whisper quiet. So I have no idea what was causing that noise a few months ago. When it happened I removed the side panel and had a listen and I thought it was the powersupply. And since then I thought it was causing a noise, but I now know that is my GeForce6 graphics card. So I was very wrong in my diagnosis (not the first time).
The New Powersupply is good and the neon light inside complements the neon light on my GeForce6, and my neon tube light that lights everything else up, in neon.