Gear horn

[stackj]

Look for a broken wire near the gear warning switches on both engines throttle bodies. I don't know of anything that would cause your exact symptom, but a broken wire there which just happened to touch your engine during the retract period would sound the horn.

Normal engine viberation could have caused the wire to move away from a shorted position just as your gear retracted. If this is the case, you will probably have this problem intermittently until the wire is repaired.