The 707 was in the cruise, at night, in 1989. Two periods of bright yellow/white flickering from inside the engine, each period less than a minute.
Ah, a great deal more info
Well it probably wasn't reverser operation
The color yellow is significant (most reports are bright white sparks)
White would go with titanium blades (being FODed or rubbing their tips against the case). Yellow goes with steel blades. If its arround their tips its rubbing (screwed up bearing) if its random then its chewing up something loose like a rivet or a stone. Common on the ground but not at cruise.