I'm not familiar with the 767, but in the conditions that you describe it sounds like it could have been a static engine run-up. When the OAT is close to zero and there's fog, snow or other precipitation some engine manufacturers stipulate a requirement to perform a static engine run-up at certain intervals / prior to setting takeoff power. This procedure ensures that any ice that has accumulated on the fan blades is shed and also gives the opportunity to check that the engines are stable with acceptable levels of vibration.
Here's an example: