Newly delivered 747 on the ramp, I was booked to ride jump seat to collect data. A crew of airline techhies transitioning to 747 were crowded in the cockpit, so I was relaxed in the upper lounge awaiting the flight time.
I heard a groaning sound - maybe hydrailics? - What were those fixerguys doing to this airplane?
I looked outside and - AHA! - A TriStar 1 km. down the ramp was emitting a large cloud of white vapor from #2. Loooonnng groaning continued a minute or more. Finally the smoke and the noise ceased almost simultaneously. Then the wing engines began the same complaint cycle.
Weather was clear, OAT near ISA.