Zazoo,
You don't say where you were actually sitting in the B767-300, but just aft of the wing on the left side you would be above the Air Driven Hydraulic Pump (ADP). This is only supposed to operate in certain conditions such as when very high demand on the centre hydraulic system (such as Flap Operation simultaneously with Landing Gear operation) causes centre system pressure to fall below 1900 PSI. In certain fault conditions, especially a defective pressure switch, the pump may operate when it is not really needed. If it WAS running you would certainly notice the unusually loud noise as would other passengers. This is not a serious fault and there is no risk involved, it just wears out the ADP faster!
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Through difficulties to the cinema