I opened it up, and the noise was coming from the Heatsink/PSU region.
I switched it on without dusting and the noise continued...I thought possibly it could be because the position it was in.
Anyway, the dusting appears to have worked! I was shocked how much was in there, there metal shield on the heatsink had hard bits of dust welded onto it! And the fan for the Video Card was covered...shocking stuff.
I'm currently considering moving my PC to a different position in my house anyway, and when I do it'll be alot easier to pull out and clean.
Thanks for the help guys - and thank God it wasn't my hard drive!

Another small question in relation to me moving my PC. The PC will be the otherside of the room from the phone socket. I have a USB ADSL modem which plugs into a phone socket. Could I get an extension cable to plug into my Modem that goes to the phone socket -around 5meters? If I could is it likely to cause any problems as this would laso be pluggin into a filter?
Thanks again!
