September 2011 MacBookAir4,2 1.7 GHz White noise even when sound turned off

Since new this MacAir has had a white noise like a tv not tuned in even when sound is off. Gets worse as it gets warmer. Seems to be getting worse then ever.(summer). Usually connected to a Thunderbolt Display and makes noise only when display is attached. Disconnect the display and the noise goes quite after a few minutes till display is again attached.

Mountain Lion OS X 10.8.2 (12C60)

I think it is the Cooling Fans making the noise and the Thunderbolt connector gets hot after a few minutes.

Rom Ver MBA41.0077.B0F

Regards

Barry