B737 Undifferentiated or Other Model
Confusion; Human-Machine Interface
Aircraft Equipment Problem Less Severe
Flight Crew Rejected Takeoff; Flight Crew Returned To Gate; General Maintenance Action
Aircraft
We complied with all procedures and checklists including throttle bursts during the Before Takeoff Checklist. We got no Takeoff Configuration Warnings at any time. I was the Pilot Flying. I advanced the throttles to about 40-50 percent N1; so they were matched before I set takeoff power. No horn. When I set takeoff power we got a muffled horn sound which we did not immediately recognize as the Takeoff Configuration Warning horn. The sound was muffled and weak almost like it was powered by a dying battery. The horn did not sound normal. There was no associated warning light with the muffled horn. We rejected the takeoff at about 40-60 knots; notified Tower and exited the runway. We checked our configuration and everything was set appropriately; including the flaps set at 1 per the performance data. We still got the warning horn during throttle bursts. We reran all checklists to see if there was anything else we missed and still got the warning horn. We taxied back to the gate and made a Maintenance write up.
A B737 Takeoff Warning Horn alert volume was so low the crew did not identify it until takeoff thrust was set. When the warning was finally identified at about 50 knots; the takeoff was rejected and the aircraft returned to the gate.