The oven motor is pretty common to break down on a 6850, I've replaced multiple.
To confirm that it's the oven motor set the oven to off in the CDS software (probably Chemstation). Check if there's no sound.
Turn the oven to on and set the temperature to 25 deg.C in the CDS software.
Check if the sound is back.
If there's excessive sound when the oven is cooling it could be that the motor needs to be replaced.
To do this you need to remove the entire back panel from the GC.
James_Ball wrote:
If the 6850 is like the 6890, there is also a small fan near the injection port that helps cool it. It is behind the port near the split vent trap. Normally it is just a standard computer cooling fan, but they can get noisy and especially if something has move against the blades.
The 6850 doesn't have a big inlet cooling fan like the 6890, 7890 or 8890. There's a smaller inlet fan to the right hand side. This often breaks down but because it's so small it's unlikely to make any sound.