What was found
ORD - FLT 3900 - DURING CLIMB-OUT A SHARP THUMP WAS FELT IN THE FORWARD PORTION OF AIRCRAFT AND SOUNDED LIKE SOMETHING HAD CONTACTED THE AIRCRAFT. ALTITUDE WAS 23,000 FEET AND AIRSPEED 250-260 KNOTS, SOUND AND THUMP WAS FELT AND/OR HEARD AT LEAST TO ROW 6 IN CABIN. EVENT OCCURRED REPETITIVELY, BUT AT NO DISCERNABLE INTERVAL. A BRIEF YAW AND CONCURRENT DROP IN SYSTEM 1 HYDRAULIC PRESSURE WAS NOTED WITH EACH THUMP EVENT, SEEMED LESS FREQUENT AT LOWER AIRSPEED, BUT UNAFFECTED BY AUTO PILOT AND YAW DAMPNER DISENGAGEMENT, LOWER ALTITUDES, OR AIRCRAFT CONFIGURATION.