As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CONTINUOUS HORN SOUNDED FOR 3 MINUTES BETWEEN 3000 - 6000 FEET THEN WENT SILENT ONE MINUTE & CAME ON AGAIN. NO ASSOC LIGHTS OR AFTER INDICATIONS. SUSPECT CABIN ALTITUDE WARNING SWITCH AT FAULT. R & R CABIN ALTITUDE WARNING SWITCH S00128 IAW AMM 21-33-04. OPERATIONS CHECKED GOOD, NO FAULT NOTED.