As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
HF 1 ACTIVE FREQUENCY SWITCHES ON ITS OWN INTERMITTENTLY WENT FROM TUNED FREQ OF 10057 AND CHANGED TO 20054 5 TIMES DURING THE ETOPS PORTION OF FLIGHT. GROUNDED - YES. UNBALE TO DUPLICATE, ACCOMPISHED HF OPERATIONAL TEST REF B737-900 AMM 23-11-00-710-801 TEST PASSED. ACCOMPLISHED RTP OPERATIONAL TEST REF AMM 23-12-41-400-801.