As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER TAKEOFF, "TOO LOW" AUDIBLE WARNING WENT OFF WITH BOTH RADIO ALTIMETER FLAGS IN VIEW AND GPWS LIGHT ON. DUE TO FUEL AND RVSM ISSUES THE CREW RETURNED TO DEPARTURE WITHOUT FURTHER INCIDENT. MX REPLACED RADIO ALTIMETER TRANSCEIVER. OPS CHECK GOOD IAW MM 32-42-0. ACFT WAS RETURNED TO SERVICE.