Best trying to determine if it come's from the speakers, electronic
or if a mechanical noise, hard drives can cause this problem and so
can CD ROMs.

I have known switch-mode power supplies cause this..... and there's
one right inside - the inverter for the backlights, this little
device steps the 12V dc up to around 1,800V, just wondering if it's
something arching over on the HV side, whether this would affect the
light from the CCFL's would depend on the nature and severity of the
fault.

Once the lamps have ignited the running voltage drops to around
1,400V.


In tv's and videos I have used a length of plastic tubing as a
stethoscope to locate the area and usually locates the component or
mechanical gear wheel.

Davy